I'd like to share some practical advice:
1. vendors will try to rip you off. anyone selling you anything/charging you for a service/etc will charge you many many times the going rate. don't be fooled, and don't feel guilted into paying what they ask for. you should always negotiate, and never hesitate to walk away.
2. i know everyone wants to enjoy local cuisine, but you have to be smart about it... i really don't recommend eating street food... chances are you'll get food poisoning at least once on your trip, and it's not a pleasant experience by any means. obv you should drink only bottled water, and i recommend avoiding any fruits/vegetables that have a lot of *water* in them, as the little viruses/bacteria/etc that make you sick are in the water.
3. women- don't make eye contact w/men. it' s a cultural difference- they'll think you're hitting on them and this can lead to problems. if you *do* make eye contact, act bitchy. trust me, you want to do this... i didn't listen the first time i went as a teen and i definitely had men following me home.
4. about the beggars, esp the children- it's heartbreaking to see them, i know. as others have said, if you do give them money, they will remember you and come back to you. and if you give them money, you will get *mobbed*. if you really want to help a child out, i recommend buying them some food from a street vendor or someplace nearby and making sure they eat it. just my opinion.
5. it's most likely that people will be nice to you if you're foreign. appreciate it :)
6. for those people who want to go to learn ayurveda/yoga/etc... just be aware that many of those centers are designed expressly for tourists and market things differently...
I hope some of that's helpful, although I doubt if any of it is news to you. Nothing beats sipping a nice cup of chai while sitting on the sunny roof, letting my hair dry. And the earthy smell that is so distinctly Indian is just wonderful. Mmmmm. I love it!