-Learning some of the local venacular can save you embarrassment/trouble. For instance, if you are an American travelling to Britain, New Zealand or Australia (I think), do not use the word "fanny." (And for that matter, do not wear fanny packs.) It's rude. Also pants = underwear. And in Jamaica and many of the other West Indies, the term "native" has racial connotations. Say islander instead.
-If you are in country with a different language, never assume that they don't know your language. I worked retail while in University. Two Austrian tourists came into the shop and started saying all sorts of rude things about Americans in German, assuming that they wouldn't be understood. They were shocked when I addressed them in perfect German. (Which I have since lost!
-Learn about local customs. We ran into this obnoxious American family in Rome a few years ago who were angry because they couldn't order lunch during siesta and went on a tirade about Italians and Spaniards being "lazy." (The sad thing is that the people who need to be reading these etiquette lessons are not the ones who are likely visiting this board!)
-Do not lump everyone of one nationality together or assume that individuals speak for everyone of X nationality. Two American friends were badgered on their Irish honeymoon by a sanctimoneous German who felt that the US wasn't doing its part environmentally.