I've gone with -- and taken -- many people to India who have never gotten "Delhi belly". There are a few rules to abide by. Always use bottled mineral water from a good source; and use it for most everything including to brush your teeth and to clean your tootbrush. Keep your mouth closed when taking a bath/shower and wash your mouth and face with bottled water after you're done with the shower. Wash your hands with soap frequently and dry them completely or do the final rinse with bottled water. Don't eat uncooked food and avoid dairy products unless it's been boiled or is part of something that has been well-cooked. Avoid frozen foods and sweets except at the best of the best hotels. Don't eat salads. Be careful with fruit. Eat well-cooked food only, and that too only eat "hot food". All from good restaurants. Avoid street-side vendor foods, cooked or not. Wash your utensils with bottle water or wipe them down with antibiotic wipes and then wait for them to dry completely. Travel around in A/C cars with the A/C running and the windows closed. Wear appropriate clothing indoors and outdoors. Keep your hands away from your face as much as your can.

Do I follow all those things myself? Not all of the time, nor even most of the time. But those steps certainly do work.