I am asked all the time about the “best” appeal letters. Everyone who asks is pretty sure that there is some science behind direct mail pieces. And there is science behind it. The science, however, is specific to each mailing and each agency.
The only way to know what the “best” appeal letter is for your group is to test it. Send out two letters, one is the “control” — the one you have used before. The other is the test with some new conventions (eg: full color envelope, 4 page letter, etc.).
In the link above, the author Willis Turner walks you through several tests and the results. If you really want to know what works. Test it.
By the way, SOFII is an incredible resource. They have an extensive library of fundraising letters, nonprofit advertising and other articles.