I'll try to break it down by what, for convenience sake, you might want on a short trip, and what not to bring you're going to be staying long enough that it's pointless to bring a year supply of X because it is available if you know where to look.
DISCLAIMER: Please keep in mind most of my knowledge and experience is specific to South India, and in particular Tamil Nadu, and things might be, in fact probably are, different in the North. Also, this is my personal advice based on experience, I am NOT a medical professional, and you should always consult your doctor on anything medical-related before traveling abroad.
1. Toiletries and over-the-counter medical stuff
2. Mosquito Related
3. Guide books
5. Assorted Useful Gadgets
6. Gifts to bring
1. Toiletries etc.
For the traveler on a short trip, go ahead and bring a complete set of all your preferred toiletries, your own soap, shampoo, conditioner, toothpaste, deodorant etc. You should probably also pack a small first aid kit. Most of these things are available here in India, though not always in the brands or forms you might prefer. But if you're going to tour around and try to see the sights, you don't want to waste your time shopping to find toothpaste, so you might as well bring your own.
For someone coming to India to stay a long time, you can actually get almost everything you would want here including familiar American brands. This is totally different than it was in 2001 when I first started coming to India. Now you can get all sorts of brands of shampoo AND conditioner (used to be you couldn't get conditioner anywhere).
A couple of items that are hard to find, and especially people are picky about what they use: DEODORANT, especially stick deodorant, and also surprisingly, any kind of herbal deodorant. There are tons of sprays, usually heavily scented. Occasionally in a store with imports you can find some brands like men's Speedstick, but if there is something in particular you use, you may want to bring a big supply of it.
For women: Tampons. Pads are widely available, and for some reason, OB are becoming more available, but if you prefer anything else, you should probably bring it. Also, yeast infection treatments.
Some things are harder to find: Cloth band-aids. And band-aids in shapes other than long strips. I bring a big supply of the "finger" band-aids, or off brand varieties of the same. A good, well stocked first aid kit is always a good idea.
In terms of medical stuff: Some of the most familiar medications are hard to find, or seem to have different names. I bring Ibuprofen, Pepto-bismol (or generic), Immodium (or generic) and Sudafed not "PE"(the real pseudophedrine that you have to sign for works best for me). These items (or compounds) may be available here, but not under ANY name I've ever asked for at a "medical shop". Which, by the way is what they're called here at least in the South, if you're ever in need of one. There are of course different versions and equivalents of the kinds of things listed above: Paracetemol is the same as Tylenol, several things available for upset stomach, and various cold remedies. My reason for bringing the American versions is that when I'm sick, I'm also comforted by knowing what I'm taking, and knowing that it'll work, because I've taken it before.
You probably don't need to pay for a full prescription of something like Ciprofloxacin which doctors sometimes recommend getting in advance. It's a very good, very strong antibiotic, and it is great if you have a bad bout of diarrhea, dysentery, or a urinary tract infection, among other things. However, it IS available in India, for much much cheaper than it is in the U.S., so if you should need it, you can get it easily.
One last piece of advice from experience: If you do see a doctor while in India, and are prescribed something you don't recognize, it's probably a good idea to look it up online, and see what it is before taking it. If you have any drug allergies, this is especially important, because it may be something you're allergic to under another name. Even if you don't have drug allergies, it's a good idea to know what it is before you take it. This is a (sad and funny) story for another post, but lets just say all doctors are not created equal. Some in India are among the best in the world, and others are complete hacks. A good motto in this department is "Trust but verify".
2. Mosquito Related
To take prophylactic anti-malarial drugs, or not to take prophylactic anti-malarial drugs, that is the question!! This is my two-cents. Again, I am not a Medical professional, please consult your doctor.
Malaria is an extremely serious disease , it can kill you, or be very, very unpleasant, it causes brain swelling among other things. However, Malaria is not the only mosquito-borne illness you have to worry about in India. There is also Dengue, Chikungunya, and rarely reported West Nile Virus. These last three are viruses, and Malaria is a parasite, so any medicinal prophylaxis that works for Malaria will not prevent becoming infected with one or more of the viruses.
If you are going on a short trip to India, short meaning less than a month, and if you are going to be in warmer more tropical regions (i.e. not Ladakh, Kashmir, or any part of the North in winter), you MAY want to take one of the anti-malarial medications out there. There are two main types, the quinines, and the antibiotics. Both varieties usually require you to start a week before departing, and stay on the medication two weeks after returning home. Get a prescription from your doctor, and make sure the know exactly where you are traveling, because there are specific drug-resistant strains of malaria in some regions.
Both categories of anti-malarial prophylactic drugs have side effects, and these vary person to person. Some of the serious side effects of the Quinine varieties (taken weekly), such as Chloroquine, Mefloquine, and Malarone, have reported psychotic side effects, including violent dreams, suicidal urges, and rarely psychotic breakdowns. I know of at least one student on a study abroad program who had such a breakdown, and had to be sent home.
The most common anti-biotic drug prescribed as an anti-malarial is Doxycycline. It is taken daily, and must be taken with food. Over the long term it is known to harm the stomach, and can cause increased sensitivity to the sun, making serious sunburn a risk if you are on Doxycycline. Since it is also an anti-biotic, the risk of yeast infection for women is also higher. It doesn't have the same risks of psychological effects as the other categories of drugs, but I did experience severe stomach pain after about a month of taking Doxycycline.
My personal opinion is that if you are going to be in India for longer than a month or so, it is not ideal to take any of the anti-malarial prophylactics, because of the side effects, and generally having such strong chemicals in your body for such a long time. In addition, since these drugs do NOT prevent Dengue, Chikungunya, or West Nile Virus, your best bet is to take lots of preventive measures to avoid being bitten by mosquitos at all.
I use a pump-spray (not aerosol) with deet, such as Off Family Care (or generic). I use it on a daily basis, and I especially spray the skin of my feet and ankles, the cloth of my pants around my ankles, and my exposed arms and neck. With maybe one or two sprays aimed at the clothes covering my mid-section. Even during the monsoon season in Tamil Nadu, which is a pretty mosquito-y season, this is enough that I haven't been bitten at all in the 3 weeks since I have arrived, except on the one day I forgot to put it on.
I also sleep under a mosquito net. I use a portable variety, which hangs from a line strung between two points in the wall or a single point in the ceiling. (This is way easier to set up than the 4-point variety.) It is light and pretty easy to pack, but if I was going from hotel room to hotel room on a daily basis I wouldn't want to set it up over and over again.
As for what's available here, they use a cream mosquito repellent applied to the skin called Odomos, and several varieties of plug-in diffusers which diffuse a chemical into the air that pretty much keeps the mosquitoes out of the room into which it is plugged. The biggest brands are Good Knight and All Out, and I recommend the liquid variety over the pads. You can take one of these with you from hotel room to hotel room, and plug it in as soon as you arrive (though most hotels will provide one often they use the pads, which I find are less effective). It works great overnight, even with the windows open. Some varieties are wall plugs that have no cord, just plug into the wall socket, others have a cord of about a meter, which I think is better for a bigger room, since it means you can position the diffuser sort of near the middle, or nearer to your bed, in case the wall socket is far off in a corner.
I always carry a spray bottle, tube, or wipes of some sort of repellent in my bag with me at all times. If you find yourself in an especially mosquito-y area, apply more!
Since I have known people to get both Dengue and Chikungunya, and both were quite unpleasant diseases, with sometimes long-lasting effects of joint pain, etc., I decided my plan was not to take any of the anti-malarial medications, which I think may give a false sense of security about being bitten by mosquitoes, and instead I am very careful not to get bitten at all, if at all possible. I am here for a year, for a shorter trip, I might consider taking one of the drugs.
And on to happier subjects...
3. Guide books
I personally prefer the Lonely Planet guide books, as I have found their balance of information about sights/sites, and information about food, lodging, banks and other facilities to be very useful.
Lonely Planet offers several guides to India, including an all-India guide, and several specific guides to different regions. If you're going to travel in South India alone, I highly recommend their South India Guide. It's got more detail, and a different set of accommodations than the main guide. It also gets you "off the beaten path" a bit more than the main guide.
I also hear good things about the Rough Guide to India. Especially that they give more in-depth historical and contextual information about sites, compared to Lonely Planet. However their listings of other information, such as accomodations, food, etc., are sparse.
One important thing to note is that the different guides cater to different crowds, and the Lonely Planet guide tends to cater to, and be used by the young, international, and to some degree "hippie" crowd. While the Frommer's India tends to be upscale in their selection of accomodations and the prices are more expensive than most young people can afford, so the Frommer's guide works better for more "grown-up" crowds.
I suggest going to a physical bookstore to peruse the various options, so that you can see for yourself how they are organized and what kinds of information they present. This is the best way to find what works for you.
Clothing is particularly an issue for women traveling India. This is not a particularly good thing, but it is a fact of life. What is considered culturally appropriate or acceptable varies significantly from place to place, and especially between major urban centers and other smaller cities, towns and rural areas.
Men have it pretty easy. As long as they don't wear short-shorts, they are probably fine almost anywhere. For entry into a temple or mosque they may be expected to wear long pants.
Despite what Bollywood movies show, very few women go out in public in any context, urban or rural, wearing a tank top. Some varieties of sleeveless tops, those with higher necks, going to the shoulder joint, with no bra visible under the arm MAY be acceptable in cities like Delhi, Mumbai, Bangalore and Chennai, however, they are still not common. In general short sleeves are required.
If you're coming for a short trip as a woman, you may not want to purchase all new Indian clothes on arrival, though who knows, maybe you will. If you do pack from home, it should be appropriate to the time of year and climate (look it up based on when and where you're going.) It does actually get cold in the North in winter, and VERY VERY hot in the south, especially in March, April, and May.
Still, despite the heat, shorts, tank tops, and skirts above the ankle are a bad idea. You will probably be considered to be a "loose woman" (akin to a prostitute or porn-star) wearing a tank top and shorts. You will almost certainly get pinched and groped, cat-called and harassed. I hate to say that you will be "asking for it" but in this particular cultural context, that's exactly how it's viewed. The Indian perception of western sexuality is already one that is extraordinarily promiscuous (mainly perpetuated by Hollywood movies and pornography), and if you want to avoid being labeled as such from the get-go, you should consider wearing clothes as modest or traditional as possible. This conception of promiscuity permeates pretty much the whole society, so not only does dressing respectably get you fewer gropes from men, it also gets you more respect from women. And more than that, if you wear Indian styles of clothes, they appreciate the gesture as respect for their culture, and will generally say so. It's valued as much as, or more than, learning a few words of the language.
The two major traditional kinds of Indian dress for women are sarees and salwar kameez.
To get Indian clothing you can buy "ready-made" Salwar Kameez, or have it stitched by a tailor. Many shops sell "cloth sets" which include 3 matching pieces of material which can then be stitched into a top, pants, and shawl. You can also get saree blouses and under-skirts stitched (ready-made really isn't a very good idea for a saree blouse). The shawl, or "dupatta" part of a salwar set is an essential item, it is intended to cover your breasts, and even though it may be completely transparent fabric, it is still considered "modest". Ready-made stuff is sometimes not well sized for larger Americans and foreigners, so you may actually need to get things stitched by a tailor, if you are a bigger or taller woman. Even if you are not, having things tailored to fit you, just right in all dimensions, and to your personal specifications is just a nice feeling.
Modesty is an extraordinarily important aspect of dress for women, and western clothes, are usually de-facto immodest, even if they are baggy or cover a lot of skin. If you're coming with your own western clothes to India, I highly recommend bringing or buying a shawl or shawls to cover your upper body, and occasionally your head. (To visit a mosque you will be absolutely required to cover your head.) Even for daily wear in many cities and towns, you will find that "modest", appropriate, and especially traditional Indian styles of dress will get you friendlier treatment, and less harassment.
For visiting sacred sites, temples, mosques, etc., be aware that even more strict rules on modesty apply for both men and women. If you plan to visit such places, keep this in mind. You may not be allowed inside without appropriate clothing.
5. Assorted Useful Gadgets
The most useful gadget I own is my headlamp. I have a Black Diamond Cosmo headlamp. I use it when the power goes out, and when I want to read a book in bed at night, inside the mosquito netting, and don't want to have to get in and out of the netting again to turn off the light. It's comfortable, it doesn't require a hand like a normal flashlight, and it has two different settings for bright spotlight and wider more diffuse light. I find it is an indispensable part of my life. The power goes out frequently, either in scheduled power outages, or randomly due to various glitches in the system. I have packed a suitcase by the light of a headlamp, prepared dinner, and a wide variety of other tasks. I suppose I could live without it, but I wouldn't want to. You could bring a flashlight, but I think that would be a much less practical choice.
If you are bringing any electrical devices, laptops, cameras, anything with a plug, or rechargeable batteries you may will also need plug converters. If your item (such as a laptop) has an AC adapter, all you need to do is convert the type of plug to the Indian plug. If it does not (such as hair dryers, electric razors) you will need a voltage converter. Checked the specs on the item you are carrying, and do some research. These are available in India, at some "electrical shops" but they can be hard to find, and it's usually easier to get one or two in the U.S.
6. Gifts to bring
Gifts are a good idea to bring, especially if you will be staying in anyone's home, or visiting friends or friends of friends. It's not 100% expected that you will bring gifts, but it is definitely appreciated.
Ideally you want to bring gifts that people can't get themselves in India, or that are prohibitively expensive here. This is (unfortunately?) an ever shrinking list of things, as global consumer capitalism continues to expand, and new markets are opened in India, more and more things become available here, and are no longer special or gift-worthy.
One good category is chocolates, especially fancy boxes of chocolates or chocolate bars.
Also location specific t-shirts and other paraphenelia like key-chains, mugs etc., from a home university or home town. This can get expensive if you have a lot of gifts to give, but it is definitely something you can't get anywhere else.
Lastly, anything home-made, any home-made clothing, cards, paper, art, food, etc., made by you or your family. Home-made gifts are frequently brought by Indian families to their counter-parts in the U.S., and it works both ways. Some non-commercially packaged foodstuffs may be a problem in customs, so pack it well.
It used to be that pens were something everyone asked for, and now there are plenty of good quality pens widely available. However, it is still "traditional" in small rural villages (at least in the South) for kids to ask for "school pen, school pen". So if you think you might want to just give a way a load of regular ballpoint pens, get a couple packs. You'll be immediately popular with all the kids in the village.
Wow, that was long! I hope you find it useful. If you have any questions, critiques, or anything to add, please leave a comment.