A proper VISA to enter and stay in India is a must. There are reported cases when travelers are advised non-requirement of Indian VISA by their travel agents. Practically every foreign national requires VISA to enter India. Obtaining e-Visas is a simpler process and is applicable to most nationalities.
Travelers should get properly inoculated against Yellow Fever if coming through infected regions.
It is advisable to cover yourself with travel insurance for thefts, loss and medi-claim. It's best to bring along a smartphone and buy a local sim at the airport of arrival. Indian call rates are very low and the cellular network is widespread. You may take assistance of your local tour operator in this regard. Almost everybody in India keeps a cellphone. Having your own local number will help you in tracing your local contact, driver, representative and practically everybody whom you need.
Carry proper maps of the places proposed to visit in India, as signboards are often absent. The Google map in your phone will be just as fine. Try to reach a station during daytime if traveling on your own. In any case avoid persistent touts and taxi-wallahs at airport/stations/bus stand to help you find your hotel. Always use tourist assistance desk or call your reliable local person for proper advice.
Women traveling alone in certain deserted places should avoid walking at odd hours.
Don't ever enter a temple, tomb, dargah or Gurudwara with shoes on and/or scantily dressed. One should cover his/her head with a cloth while in a Gurudwara or Dargah. Parikrama or walking around the sanctum sanctorum should always be in clockwise direction.
Do not wear black clothes while visiting a Jain temple. Leather articles are forbidden to be carried in certain Hindu and all Jain temples. Taking photograph of the deity in a temple is normally not permitted.
Participating in a social occasion or visiting a home requires conservative dress codes. Do not shake hands with ladies. If you are a male greeting Indian ladies then the gesture of saying a Hello or Namastey will make them comfortable. If they are comfortable with handshake then allow them to extend the protocol of shaking the hands first. Always pick up anything to eat only with your right hand. Take only as much as you can eat, do not leave anything uneaten over the dish.
Do not point your finger at any person. It is taken as a sign of annoyance.
While changing money, insist on getting encashment certificate.
Do not encourage beggars.
Do not checkout of the hotel in hurry. While checking out it has been noticed in some hotels, the extras are unreasonably charged which the guest hurriedly pays without cross-checking.
Do not leave your cash and valuables in your hotel rooms. Keep your cash divided in different pockets.
Take care of proper disposal of your rubbish.
Be careful of cultural and social sensitivities of the regions. There is no single rule for that, the best way is to observe and follow.
Take care of contamination of water and food problem. Always drink safe mineral water and take well-cooked food.
Don't buy antiques more than 100 years old. Selling and buying "shahtoosh" shawls is a crime. The same goes for ivory and wildlife.
Buy at genuine shops only. Bargaining is a popular practice in India and necessary too. Don't ever believe in lucrative offers of antique dealers in which they offer you to carry a parcel of some other buyer back home with your own margin described. Entire transaction should be legal and transparent so that you may claim later if dissatisfied.
Don't eat anything offered by fellow travelers on train or road travels. It might have sleeping pills. Always travel reserved class in trains.
Always chain and lock your luggage under your berth in a train. Don't keep anything valuable near the window. Always carry plenty of water, fluids in trains. A lone woman traveler may request to be accommodated near other women travelers.
Do not visit places which encourage orthodoxy, social injustice and inhuman practices (like visiting a sati temple).
Don't photograph women without permission.
Don't accept offers of visiting anyone's home unless you are confident of the person.
Use licensed guides for sightseeing.
Always use strong suitcases/baggage.
Don't tip unreasonably and unnecessarily in a hotel. This may lead to more expectations.
While traveling, don't act confused. Keep a posture of a person known to the region.
If eating in buffet, even in expensive hotels, avoid picking salads and rice. The food may become contaminated due to over-exposure. Indians prefer eating freshly prepared meals. Follow the wisdom.