Generic travel advice:
Pack as you would normally for any other trip. I won't bother mentioning paperwork, appropriate clothing, cameras and vaccinations as those are generic for practically every travel destination.
A list of good places to eat. In the context of India, this is mostly word of mouth knowledge, so ask your Indian friends, or better yet ask a question in:
Local Restaurant Recommendations
Gifts ( if you have friends or family ):
It does not matter if what you are getting them was made in India and exported to the US, it is a strange illogical ritual that Indians partake in when it comes to traveling really heavy with lots of gifts for friends and family. Even though you can get practically everything you get in the US in India in the big cities, many people in India don't realize how much parity there is.
Credit cards are not accepted everywhere. Be you are carring a moderate amount of cash. Preferably some in low denominations.
If you have electronic gadgets, be sure to get an appropriate power adapter.
Get a prescription from your doctor for any medications you are using. Stockpile them while you are in India, as they are ridiculously cheap there and they work just fine, though technically to bring them back into the US you need a prescription.
An unlocked phone:
So that you can get a SIM when you are in India. If you have an old iPhone, you can jailbreak it, or if you aren't tech savvy enough to do so, take it along and have one of a jailbreaking services take care of it for you.
There is voice and data roaming in India if you use an iPhone, but you probably don't want to use it for anything beyond the most basic SMS
Get a sim card and data service after you land.
Once you are there:
The bulk of your trip will be business, sightseeing or meeting people depending on your goals. Prepare for this as you would for any other trip, though here are some tips of what to do beyond that.
If you look Indian, you might be harassed at customs. You might have to bribe on of the customs officials to expedite this. Yes, this is increasingly rare, though plan for the worst case.
If you don't look Indian, you probably won't be bothered by customs, though in that case there are a number of other things to be careful about.
Mostly this means that you will be overquoted prices on a number of things. Try to always ask around for price lists ( not possible a some shops ). It is common to haggle.
If you are traveling with an Indian friend or have local friends there, be sure to tap them as resources.
Most parts of India are generally safe, and most crime that is there is petty crime, like pick pocketing. Still, don't go around with your passport, US drivers license and anything else important. Keep them in the safe at your hotel.
Don't drink tap water. Eating anything cooked is fine.
Sushi and beef are rare to come across in India to begin with, though even if presented with the opportunity, avoid both.
Don't try to drive. Use a taxi. You might find it odd being on the other side of the road and be a bit freaked out by people ignoring lanes, red lights and one way signs while at the same time paying attention to cows crossing the road, but you will get used to it. Try to stay as close as possible to your primary point of interest.
There are subtle and not so sublt signalling protocols employed by people driving in India
You are mostly okay with English. Even many not so literate folks in India are able to understand English.
However, Indian English is closer to British English rather than American English at times, so there are a number of subtle differences. In rare cases, these can be a little awkward, in particular "Fags" is a US is used as derogatory slang, though in India it refers to cigarettes.
If you like to read books, you can get Indian editions of books ridiculously cheap. For example, you can get a copy of Introduction to Algorithms for ~7$.
If you are on the verge of buying something, plan for it and save some money.
Simple medical procedures
You can get dental cleaning and eye exams done in India really cheap as compared to the US. The same goes for getting glasses, though first you should figure out if they have a frame that you like.
Finally, don't write answers to questions like this when your flight to India is about to start boarding.