1. If English is the only language you speak, speaking it louder and more slowly does not serve to increase understanding in anyone else. Don't do it! It's annoying and you look ridiculous. Be proactive and learn a few useful phrases before you go. Suggestions, 'exit' and 'bathroom' - you wouldn't believe how often these two words will save your bacon.
2. Be a traveller, not a tourist. That means, eat something different, embrace the local customs and the locals, do something different...and for heaven's sake quit the whining! Makes it look like you're trying way too hard to feel superior, which will make you look like a loser to the more discerning. If you find yourself beginning a sentence with 'In America we...', do yourself a favour and punch yourself in the mouth before the person next to you does.
3. Do not overpack. Pack your bags, pack your money, then take out half the clothes and double the money. Don't buy the things you see on the first day - ten chances to one, you'll see them for half the price by the fifth day.
4. Take local transportation, it's fun. Learn how to read a map. Not everyone is out to scam you but err on the side of caution and just pretend they are. You'll get good advice from bus, tram, trolley drivers so take heed. Don't take a taxi if public transportation is readily available.
5. Finally, have fun! Even in the pouring rain, the baking sun and the blistering cold. TAKE LOTS OF PICTURES...but don't post all 500 at once. Your friends have lives too, they don't want to hurt your feelings by telling you they couldn't look at it all. Plus, that kind of photo overdose is just plain mean. And the best part of traveling is returning home in one piece, tired and happy. Believe me when I say you will love the trip even more after you're home.