The Ugly American

"The Ugly American"
American tourist in 1950's Batista-era Cuba

I'm sure that you've heard the "ugly American" stereotype before. It's prominent enough that it has its own Wikipedia page. Booming voices, plaid Bermuda shorts, mocking the locals. In fact, I've witnessed this abroad (down to the Bermuda shorts) and people are more likely to remember Americans who stand out like that. I still vividly remember standing in line at a store in Amsterdam and having a pair of American women yelling at a cashier because the women were mad that they had to weigh their own vegetables. Sadly, the "ugly American" stereotype can apply to expats as easily as tourists.
Exceptions like those yelling women aside, the stereotype is not only a bit unfair, but on those occasions that it is fair, it's not always entirely the American's fault. The US is a rather insular culture. It's expensive to see other cultures firsthand so we don't get this experience except by television or movies — and no, watching every episode of Monty Python or Dr. Who doesn't count. And it's not entirely the US media's fault, either. Strange things (other cultures) are challenging and don't drive as much revenue to the profit-maximizing entertainment industry.
So, relative to other countries, you have a rich, isolated culture whose people are often completely unprepared for the outside world, right? Well, not exactly.
As it turns out, Americans aren't the world's worst tourists.

• The British are the worst tourists in the world.
• The French are the worst tourists in the world.
• The Chinese are the worst tourists in the world.

... and so on. Tourists, in general, can be a pretty obnoxious bunch. I've watched British people complain bitterly about how few French people spoke English in Paris. I watched drunken Slavic tourists throw their trash all over a park (and almost got my tail kicked for yelling at them about it. Bad idea). I watched an African lady screaming at hotel staff for not having her room ready before check-in time. I've watched Japanese tourists ... well ... no, I've never seen a rude Japanese tourist, but still ...
It's one thing to be actively rude, but it's another thing entirely if you fail to do something "polite". In Korea, for example, if someone senior to you pours you a drink, you accept it with both hands, not one. If you accept with one hand, you may give offense. How would you even know this? But if offense is taken, have you been rude for not knowing this rule or have they been rude for not appreciating your foreignness? It depends on the strength of the reaction (I seriously doubt most Koreans would be obnoxious over this breach), but the onus is on the tourist or expat because you're in their culture.
So if you're a tourist and you don't give a damn about what the locals think, go ahead and be the "ugly insert nationality here" (just don't come round our place for dinner, please). However, if you're going to live somewhere, take that extra time to get to know the local etiquette and, if not outright adopting it, at least find a way to accomodate it.
And while we're on the topic, despite widespread advice to wear a Canadian flag or something, don't hide the fact that you're an American. Most people distinguish between the actions of the government and the citizens of that country. I've had completely random people tell me I was awesome once they found out my nationality, even though they were upset at the US government. People around the world are fascinated by America and Americans (again, not universally guaranteed). That Canadian flag might just be keeping you out of some great times.
On a final note, I've discovered that sometimes the people who judge you the most harshly while abroad are your fellow countrymen (and women). As many people are sensitive to perceptions of their culture, sometimes they get just a little bit too worked up.
I was sitting on a bus in Nottingham, a small town about two hours north of London. I was speaking into my cell phone when a gentleman with an American accent turned to me and told me to keep my voice down. "We're already stereotyped as obnoxious and loud; you should be careful."
I was surprised, but apologized and explained that I was hard of hearing (true) and don't always realize if I'm speaking loudly. As he continued to lecture me on not being an "ugly American", he told me that the local university would be hosting a small Thanksgiving dinner for US expats and guests. He said I would be welcome to join him "if it's not beneath you."
Um, no.
So the stereotype pervades and it's not always wrong, but it's easy to avoid. Be polite, read up about the etiquette of your new country and you'll be fine, but don't be afraid to be who you are, either.