Christian Reformed World Missions has created the Go Guide for Short-Term Mission Teams. It contains nearly 100-pages of resources for short-term mission teams including a covenant for team members, theological background on the Reformed perspective of missions, a lots of great practical advice.
Disaster Response Services has created a great document called Planning a Short Term Mission Trip that contains tips for traveling with their agency, as well as recommended resources for planning, fundraising, and debriefing.
ServiceLink has an extensive Online Resource Center with helps for leaders like recommended books, videos, articles, and webinar recordings. The site also contains devotions for teams, litanies for commissioning services, and other tools for your team to prepare for their mission experience.
Youth Unlimited has created resources for Serve teams like lesson ideas for before and after a trip.