Buy food from the busiest street vendors you can see. Their food turnover will be rapid so nothing will get left for long. Plus it will probably be the tastiest! Veggie options over meat.
Similarly, don't eat at any place with a gleaming kitchen, it is probably not used!
Check bottled water seals - it can often be refilled.
Take a large shawl or something to cover you - even if you are dressed modestly (recommended) you will find men trying to not-so-subtly grope you as they pass. if you happen to be blonde haired/blue eyed, the odds are even higher.
If you take sleeper trains (as pp suggests) and are on open bunks - they are amazingly comfortable and have the cleanest linens I have ever seen, but... sleep with your boots under your pillow or you may wake to find them missing.
Ask your agent/driver to find you a good guide/fixer for seeing the sights. Be prepared to have your breath taken away at the Taj Mahal, even if you have seen it a thousand times in pictures.
If you have any flexibility, fit in a dawn camel ride into the Thar desert at Jaisalmer*. The sunrise is otherworldly.
(*the Golden City, to contrast with Jaipur - pink and Jodhpur - blue).