2. Get cash before you go. It never hurts to have enough money to buy meals, get into attractions, and tip drivers. You don't know what the situation will be when you arrive at your destination. When I went I Greece in 2004, all of the banks were on strike, meaning that I couldn't get money from an ATM or cash my traveler's checks (remember those?!). Since Greece was the second part of my Mediterranean vacation, and I was too young to have a credit card, this really sucked. Those kids (I was traveling with a group from high school) who had extra cash quickly became powerful.
3. Read about the destination, local customs, and attractions. Knowledge is power, and can prevent you from missing out on something or making a fool of yourself because you aren't aware of customary tendencies.
4. Learn some basics of the language. You don't have to become bilingual, but learning key phrases will help you and will make natives treat you better because they'll appreciate your effort. My broken Italian and French has come in handy plenty of times.
5. Scan your passport and save it in an email to yourself. Don't print copies and put it in your luggage! If your luggage gets lost, your copied passport papers will be inside, which is bad news. If you lose your passport, you can head to the nearest U.S. Embassy and get a replacement ASAP from your digital copy.