I found that saying 'Namaste' with a bow of the head went down well. The way you dress can show respect. The other main thing that I remember is that, in India, as in other countries in South Asia, people use their left hand for various personal ablutions, so they eat with their right hand. I was told that, in some places, to be super polite, you don't show your left hand at the table, but most Indian people I ate with would have their left hand propped against the side of the table or something. I was also told that, when you eat with your right hand, you don't lick your fingers or anything - you wait until the end of the meal and then wash your hands. Also wash your hands each time before you eat - it's important not just for hygiene but for good manners. I also found that, when people did shake hands in India, if you have dirty hands you put out your arm with your hand down, back of the hand towards them, so that they can shake your wrist instead, which was the first time I came across that.
Check out the rules for temples etc. In every one I went to, you had to remove your shoes. I took a scarf, being a woman, to cover my head with, so see if there is anything that guys have to do. Don't wear anything made of leather when you go into a temple. Join in when it's appropriate - I thought that, for instance, at the Ganga Aarti ceremony in Varanasi, it might be disrespectful to join in the clapping etc. because I'm a tourist, but I was told firmly by a few people that actually they would find it respectful if I did.
If you go to Nepal, when you're taking the change from something you've paid for, a lot of people put their left hand on their right elbow while they're putting out their right hand to take the change, which, funnily enough, is the same thing that they do to be polite in Ethiopia. I have no idea how the two countries ended up with the same practice.
Finally (unless I remember something else and come back later), there are lots of tourists in India who don't trust anybody who approaches them, because they've read things telling them to be careful of conmen, and, understandably, they get nervous, particularly if it's their first time travelling. To be defensive to everyone who talks to you, though, is very disrespectful, particularly since you are a visitor to their country. It sounds like you've got the travel experience to relax, make friends, and make the most of your trip!
