The October issue of Budget Travel notes the 10 most egregious mistakes many Americans make that contribute to the "Ugly American" stereotype overseas.
Among the faux pas: being ignorant of "local rhythms" (i.e. rushing the waitstaff in a country where dinners usually take three hours), making "careless judgments" (out loud!) about another culture, and not dressing respectfully. That last one always reminds me of a flight to Switzerland in which I noticed a couple waiting for their luggage while decked out, head to toe, in red, white, and blue, with star-spangled bandanas to boot. (Seriously, I can't make this stuff up.)
Of course, sometimes our "ugly" reputation precedes us. When it seems as if nothing you can do will win over the local crowd, there's always the old fallback plan—tell them you're Canadian.