THere are some good books in the library on fundraising letters that I used when I started doing this kind of thing. However,I now write two or three letters---one personal and handwritten on my own engraved stationary and one typewritten on foundation letterhead that is appropriate for their records. Then I draft something from our president and the original contact person. THey revise and then I put it on our letterhead and send theirs out.
In the first letter from me, I tell how their gift will further our mission, but mostly I talk about the people who will benefit from their gift and how it will affect their lives, then I tell them how excited the staff is about the prospect of delivering the kind of service their gift will make possible.
I tell them that an official letter recognizing their gift is being prepared, but that I just had to write to them personally, because of their wonderful gift and how thrilled I am.
The president of our board and the person who was our original contact person also write letters, often drafted by me. It is important that they hear from their peers.
I hope this helps,
Imagine my delight when I returned from two weeks vacation to find your wonderfully generous gift among the pile of letters on my desk!