Always put a date at the top of your prayer letter (can be just the month and year).
Some type of greeting is recommended.
Always start on a positive note.
Update your readers/supporters on how your church is doing.
Give a personal soul-winning experience, not just corporate details.
Reference new ministries that have been started.
Always, always, always mention your family in some way–personal, ministry-related, etc.
Include prayer requests, which can be in list format or paragraph format. Unless a prayer request is extremely urgent and the main reason for the letter you are sending, prayer requests should probably be put at the end of the letter.