There's a distinction to be made between prevention of GI upset before the fact and treatment once you have it. With regard to the latter, I'm going to politely disagree with Snakeboy's post: I carry cipro as he does, but a lot of the most common stomach bugs are parasites, not bacteria, and they're better treated with metronidazole (Flagyl) and its close relatives. This includes, for example, giardiasis and amoebic dysentery; I'm personally acquainted with both.

In some locales, strict avoidance of uncooked foods is a good idea--South Asia is the obvious example. I once got giardiasis eating salad in a five star hotel restaurant in Katmandu, mistakenly thinking the luxurious setting ensured proper food handling. In other places I've eaten uncooked food without incident--North Africa, much of South America, the Caucasus. But people frequently get sick (and sometimes die) of food poisoning here in the USA, so there are no guarantees.

If you routinely eat a lot of salads at home as I do, it's a hard habit to break. Fortunately, there are lots of ways to eat veggies and produce which don't expose you to microbes; lettuce doesn't cook well, but spinach, peppers, onions, tomatoes, broccoli, even cabbage and [insert your favorite cooked vegetable here] certainly do. Usually there are sauce or stew options, and in a pinch I'll resort to overcooked, limp veggie sides for the sake of better nutrition.

I also carry multivitamin/mineral supplements, and I sometimes remember to use them.