Here's good copywriting advice from the Word Wealth blog: 7 Time Tested Tips for Making Your Advertising Work. Most of these make as much sense in fundraising as they do in advertising:
- Tell a story.
- Use cliches.
- Never write any marketing pieces if you do not believe in what it is you are selling or trying to accomplish.
- Don't try and be cute or sophisticated.
- Write to a grade 7 reading level.
- Pretend you are sitting around a fire with one of your best friends.
- Always use real postage stamps on your envelopes – never metered mail.
On #5: I'd aim even lower: 5th or 6th grade level. It's not easy to achieve, but the readability bonus is high.
I can't vouch for #7 in fundraising contexts; I've seen virtually every type of postage treatment win in various tests.