There are several components to a good support letter. Every letter, whether hand-written or typed, should have each of the following:

I. Introduction: A
friendly greeting, followed by a brief discussion of recent happenings
in your life, and how God has been leading your heart in to ministry or
missions.
II. Description of your ministry: Describe the opportunity you have with AIM and a little of the ministry you will be involved in. Mention
the training you will receive, the opportunities for ministry, the
discipleship you will experience and the opportunities for spiritual
growth.
III. Vision for ministry: Here
you will share your desire to make an impact on the mission field or in
ministry and how this experience will prepare you for it. Be open and honest, sharing even some personal spiritual growth or other goals for yourself.
IV. Request for support: Now that you've enraptured them with your ministry, allow them to share in the opportunity. Remember, this is a ministry to them. Ask for their prayers. Also
tell them the specific amount you need to raise and break it down into
realistic, bite-sized chunks of the total. For example, if you have to
raise $1,000 tell them if only five people pledge $100 per month that
is half you money. Explain the monthly pledges and challenge people to give in that way.
V. Summary and benediction: Close up with the potential for change in both you and the people to whom you will minister. Tell them of your anticipation and feelings. Finish with a blessing!

More tips on writing a quality support letter:

1. Be brief! Write, rewrite, pare down, and streamline.
2. Be visual! Include pictures in your letter. You can get any picture off the AIM website.
3. Be excited! Include your passion for the lost. Be excited about your desire to learn and grow through this experience. Nobody wants to read a drab letter.
4. Be correct! Have a friend double check for grammar and spelling errors. Ensure that the letter has a nice flow and feel to it. It should leave a good taste in your reader's mouth.
5. Be yourself! Superficiality is out; real is in. Don't impress anybody with your eloquence or spirituality. They will support you because they love who you are.

Have fun!