Avoid eating fresh salads when traveling outside of US. The water used to rinse the vegetables and fruit is suspect.
I'd say this rule applies to some places, but not all. Salads are totally okay to eat in Europe and are generally served with everything. And in Europe, the tap water is just fine to drink, even though Europeans generally prefer bottled water (particularly sparkling), and that's all they will serve you in restaurants, unless you specifically ask for tap water.
The exception to this (the only one I'm aware of) is in Turkey, where they will even tell you not to drink the tap water. There may be other places in Eastern Europe too, where it's not advisable to drink the tap water. Be sure to ask if you're not sure.
My rule for avoiding illness when traveling is to slowly integrate the local foods into your diet. I'm not saying to eat at McDonald's for every meal, but try to eat some items that are familiar to your digestive system...and try to eat very simply, and very slowly and gradually add the local foods into your diet so you give your stomach time to acclimate to them.
I say this as someone who has had intestinal problems since birth, and lives in a land where greasy, heavy food rules all. <img src="/images/graemlins/tongue.gif" alt="" />