Women-only please...
If you're traveling by train in second class/no airconditioning, request a berth in the "ladies' compartment." This is an area of the train compartment restricted to females, and men respect the privacy of women traveling alone in that section. If there are no berths available in the ladies' compartment, it's not the end of the world. But remember that it's very uncommon for a woman to travel alone in India and you might experience hassles. My advice is -- just be firm, and avoid having conversations with men who are traveling alone - this is generally a sure invitation to sexual harassement.

Families, on the other hand, are a great source of conversation - not to mention the invitations to join them when they open up their home-cooked meals. Don't think that you're being polite by refusing their invitation to eat with them - you're not, they really want to feed you.

Ed. note: I suggest you really trust your gut feeling about any strangers offering food. If you're comfortable go ahead and enjoy the goodies. However, it's worth mentioning that solo women travellers have been drugged by accepting food and drink from strangers who befriend them on trains. They wake up to find their money and belongings gone.

Air conditioning and locked doors, please...
You'll have a much more comfortable journey if you travel by second class a/c (air-conditioned). It's about double the price of traveling non-a/c, but still very cheap by Western standards. And the compartments are much, much cleaner. Also, if you're traveling a/c you can often get a private compartment (coupe), which can be a blessing. Just remember to lock the door when you're inside; otherwise, people will sneak in from the cheaper compartments and the conductor is absolutely useless at getting someone out of your compartment (conductors are usually easy to bribe). Generally, if you find yourself with unwelcome visitors you're stuck.

Lady travelling alone, please...
If you find yourself alone in a compartment full of men, it is perfectly acceptable to switch with a man who is alone in a compartment with women. Just ask nicely, using the terms "lady traveling alone", and "gentleman"; he'll usually agree.

Don't bargain with a poor man...
Remember that coolies (unskilled laborers) and rickshaw drivers barely make a subsistence living. Yes, bargaining is a way of life in India. So, bargain with a richer merchant instead of a poor man who carries heavy bags all day, or with a skinny driver with a life expectancy of 40. Who cares if he's charging you a little more than he would charge an Indian? It's only an extra 50 cents for you, and for him it's a meal to feed his family.

When you're invited home...
I met some wonderful, warm women in India who freely invited me into their homes. Do accept such invitations - it's a lovely experience to be a guest in someone's home, and you can learn a lot about India that way. If you'd like to bring a gift, fruit, candy or a little present for the children in the family are highly appropriate.