1). Take a Preview Trip. Some of the mission trip leaders should travel to the ministry sites a month or two in advance to see what you'll be doing and make any necessary adjustments. Take pictures, come back excited - become contagious about the upcoming trip.
2). Create a Daily Schedule. Everyone should know what they're doing everyday. You should wake-up in the morning and know where you'll be working, how you'll get there, where you're eating and what supplies you'll be needing. Wasting time wastes your mission trip.
3). Come Prepared for Unplanned Expenses. A flat tire, an expensive restaurant, helping others in need, having to buy extra supplies. There are unplanned and unexpected expenses on every mission trip. Make sure teenagers bring spending money - they always want to buy the most.
4). Be Prepared to Share the Gospel. Memorize the Romans Road to Salvation or the ABC's to Salvation. The purpose of all mission trips is to lead others to Christ - everyone should come expecting the opportunity to see others get saved.