I'm 17, and I work as a Wal-Mart cashier (24 hours a week) and I go to school full time. I recently developed tennis elbow, and have muscle pain in my shoulders, upper, and lower back. What can I do to ease the tension on these muscles? I thought about a better mattress (I sleep on a futon), but I'm not sure what the best solution is. I have good posture, it's just that carrying a heavy backpack and scanning/bagging hundreds of items a day is taking a toll on my back. Suggestions?