How to prevent getting montezuma's revenge in tropics?

Food/water safety. Drink bottled or purified water. Make sure cooked food is hot. Peel fruits and vegetables. Bring an antibiotic to use in case you get sick (which antibiotic depends on where you're going).
Watch at you eat... Most people get traveler's diarrhea from water sources, but food will make you sick as well. Always use bottled water or water purification tablets. Do not eat uncooked food. Go to your local travel clinic for country specific vaccinations. Have fun!