If you are serious about serving long-term in missions, start investing a portion of your time in activities (whether formal or informal) that will help prepare you for mission service.
The Local Church as a Training Ground
Some of the best preparation for "the field" is exercising your gifts and ministering in your local church and community. Seize opportunities to disciple, share your faith, and teach the Word of God. Ministering at your local church gives you experience working with the Body of Christ and a frame of reference for when you are planting churches or working with national pastors. Leading or being part of a small group geared towards discipling new believers can give you broad exposure and experience with the kinds of things you may be involved with serving overseas. If your church doesn't have such a small group or Bible study, consider working with church leadership to start one.
Build a Solid Biblical Base
Some mission agencies require formal Biblical training others do not. Regardless, knowing God's Word and how to teach from His Word are critical to most missionary tasks. You can get this kind of training through short-term Bible schools or formal accredited universities.
Short Term Bible Schools
Formal Bible Training
If you are seeking a formal education and degree program, you should seek an accredited four year college.
Learn Mission Strategies, Current Trends, and History
(Also known as: Intercultural Studies or Cross Cultural Services)
Whether you hate it, love it, or fear it--support raising will become a regular part of your life if you are going to be a missionary. Do yourself a favor and take some time to gather input about the process, the approach others have taken, and most importantly what the Bible teaches about it. You can read books on the topic or attend a hands on training seminar. Once you join a mission agency, they are liable to give you training in this area as well.
The Body Builders - Boot Camp for Personal Support Raising
An intense two day training seminar to help Christian workers get to their ministry assignment quickly and fully funded. Learn to avoid pitfalls and put to rest your doubts, fears, and questions.
» Funding Your Ministry Whether You're Gifted or Not by Scott Morton, Dawson Media
» People Raising by William P. Dillon, Moody Press
» Getting Sent by Pete Sommer, Intervarsity Press
» Friend Raising by Betty Barnett, Youth With A Mission Publishing
Find other more training resources at MissionFinder.org.