As an anthropology nut, I must tell you the single most important thing you should do before traveling overseas is to learn a little bit of the culture. Throw in some language too and people will be nicer, you’ll have an easier time getting around, and the understanding you gain will give you an appreciation for the country and people you encounter.
Get a book on the place you plan to go. Until then I found a nice little list of faux pas (things that will rub people the wrong way) across the globe. I selected the following countries for a variety of reasons, but you can check out the entire list here. Of course, nothing is better than the real thing, if you have anything to add, we’d love to hear it.