My G.I.'s Nurse Practitioner swore by a probiotic called "Culturelle" (my spelling may be slightly off). I tried a 5-day supply, but didn't notice anything different. It didn't make me gassy, though, like the acidophilus did. Still probiotics are good for you, even if you don't notice any immediate result. The N.P. said that she took the Culturelle religiously for a week before a trip to Mexico and for a full three weeks while there and she was the only person in her party to not get Montezuma's Revenge at any time.
Yogurt works too if you don't want to take another pill, but you really need to get the more expensive plain organic kind because they have more cultures--I have noticed--than the cheap-o kind that comes in super-sugery single serving cups. Plain yogurt is very tart and not very good in my opinion, but you can always self-sweeten it; I add sugared granola to mine to make it sweeter and crunchy. With the full fat stuff, a little of it goes a long way.