If you are from the US or Western Europe, nothing can prepare you for this, and no experience can substitute it. My advice:
1)Spend as much time there as you possibly can. Two months is a good start.
2)Learn as soon as you get there how to barter- and be fearless about it or you will get ripped off
3)be kind to people and talk to everyone- and learn a few phrases in Hindi- try to be as sensitive as you can be to the culture.
4)learn to ignore the people who aggressively sell you crap
5)if you are a caucasian woman, I would recommend going with a friend. (of any race & gender). I went alone and the attention started to get to me.
6)don’t fear the street food. Indeed, it is one of the best parts of the trip. Fried bananas in Fort Kochin are NOT to be missed.
7)Avoid the beaches in Goa unless you really like smelly clueless hippies