Read a magazine article recently that suggested yoghurt to prevent diarrhea. Last year in Mazatlan I got a bad case of Montezuma's Revenge.
This year I began eating yoghurt everyday for about a month before my trip. I then ate it in Mexico once or twice a day, everyday. I ate all of the same foods that I ate on my trip last year but I did not get any diarrhea this time.
The article said that yoghurt, with it's active cultures, encourages the growth of healthy bacteria in the bowel. The predominance of the healthy bacteria prevents the growth of the bacterias that cause diarrhea.

I just do the following. Take 500mg of Ciproflaxin and then an immodium. Through years of experience I've learned that immodium or pepto by itself will not work. You need a powerful antibiotic (cipro being the most affordable in SEA) to kill the bacteria that's causing the problem in the first place.