First, you need a good backpack. I used a Jansport bag that had a detachable day bag on the front. This comes in handy for day excursions. Make sure the backpack is comfortable, sits well on your shoulders and has a hip belt. The hip belt will take a lot of pressure off your shoulders and back.
Once you have your bag, it is time to decide what to pack. This is the hard part. I recommend that you lay out everything you want to pack and then take half of it away. It's also good to fold your clothes and pack them into food storage (ziplock) bags. This will keep them from getting wrinkled or wet. The bags will also come in handy along your trip. When I packed, I brought:
Just make sure you don't pack too much or you will regret it. Plan on doing a lot of laundry in sinks. You might want to bring some laundry detergent. Remember, you can buy practically anything you need along the way. Try to use travelers checks and an ATM card in case one gets lost or stolen. A calling card is also handy to calm worried parents.