Travel is important. It's edifying; it's educational, and it makes you happier than buying things does. But navigating other countries can be tricky. Get on international forums online, and you may learn that Americans aren't the hardest foreigners to spot. So how does a person be a good tourist?
Don't be stupid.
For whatever reason, it seems to be a trend lately to take your clothes off at UNESCO World Heritage sites. Kids, don't give into peer pressure. Just because all your friends are stripping down at the Roman aqueduct at Pont du Gard or flashing people from the top of the Statue of Liberty doesn't mean you need to be doing it, too. Sure, the Great Wall of China is exciting, but it's not that exciting.
Do some homework.
Learn how people dress where you're going. Americans tend to be extra casual, and if you're the only person in cargo shorts, you're going to feel like a 12-year-old. Yes, being yourself is important, but you're more likely to act like yourself if you're not busy being super self-conscious about how you're dressed. While you're at it, learn a little bit about the local religion and culture. Make sure none of your hands gestures are hideously offensive where you're going. There are some places where you can get arrested for disrespecting the wrong thing. If you can't be nice for niceness's sake, do it out of self-preservation.
Learn the language.
It's a common refrain in the United States to hear someone say, "Well if they're gonna come here, the least they could do is speak the language!" If you agree with that statement, then follow your own advice. If you don't agree with that statement, then consider this a further appeal to your sense of multiculturalism: pick up a phrasebook. (Or get on Google Translate; whatever.) Nobody's expecting fluency from a tourist, but you should at least make a visible effort. Depending on where you go, people are probably going to know some English anyway, but you never want to be the person who expects or demands that. Remember—our country is huge, and we're used to traveling thousands of miles and still hearing the same language. It's not like that everywhere, and people who aren't American take multilingualism for granted.
Nearly everyone who travels overseas (and certainly people who move overseas) has a moment of weakness and needs the familiarity of an American burger. That's understandable. But if you're not going to try any of the food, why did you travel all this way in the first place? Food is a huge part of a culture. Not only is it respectful to branch out, it'll make for a more fun experience
Be willing to get lost. Find local record stores, and buy music you've never heard of. Talk to the stranger on the bus wearing your favorite band's shirt. Learn about daily life; learn what restaurants or pubs are the best. Try things that make you nervous. Have things that you want to see and do, but don't schedule yourself so tightly that you're running around with your nose in a map trying to get to the next thing. Let yourself experience fully the places you choose to visit. Spend enough time there for it to really matter, and leave room in your schedule for things to surprise you.