Writing a letter asking for donations is not as hard as it appears at first glance. There are some useful techniques, to be sure, but it is simply another form of marketing. Recognizing it as such will allow you to write more successful donation letters, and you will be able to better provide value to your charity organization. The first donation letter will take a half hour to an hour to write; future letters will take less time.
Plan what you are going to say. Don't try to "wing" the donation letter. Brainstorm and organize the structure of your letter.
Figure out what your purpose is. Do you want a one-time donation, or are you looking for a long-term donor?
Once you have figured out your purpose, brainstorm ways by which you can show the recipient of your letter that donating to your organization is in his self-interest. For example, show how your organization will help his community.
Allow room for your recipient to mull it over. A good way to do this is to attach a "keep-in-touch" card at the bottom of your letter. This way, the potential donor can opt into follow-up letters from you and donate at a more convenient time.
Write the letter. Keep the ideas you created during the brainstorming sessions in mind as you write it.