While "manners" are often used as an excuse to be rude (as in You inferior person -- you're holding your knife wrong), normal, commonsense manners can really help you do a good job of thanking your donors.
Clairification takes a look at polite thanking of donors at What Would Miss Manners Say? Thank You. Miss Manners would advise:
- Always address your friend by their given name. A thank you is not a form letter.
- Include a specific description of the gift your friend gave you. A thank you should be personal, and show the giver that you know them.
- Describe how you'll use the gift; let the giver know why you loved the gift so much,
- Use "I." The letter is coming from you; not from "we."
- Do not be rude and describe how you could've used more.
- Do not solicit your next gift.
(I agree you should not use your thank-you message to solicit a gift, but you absolutely should make it easy to give another gift -- include a reply coupon and a return envelope.) Remember, it feels good to give, and for many donors, it feels best while you're being reminded of the power and importance of your giving.)