1. The word you want to use is “you.” Stay away from first-person singular — “I,” “we,” “us.”
2. Don’t save the best for last. State the primary benefit right away.
3. People don’t care about features. They care about how they’ll benefit from them.
4. Say “lose 30 pounds in 30 days,” don’t start with a story about how great they’ll look.
5. “Free” is the magic word. Create something that’s free — but don’t make it a conditional free like, “buy 12 ounces, get 4 ounces free.”
6. Have a call-to-action. Posting a 1-800 number isn’t the same as saying, “Call today while the color you want is still in stock.”
7. If your product works like you say it does, why doesn’t the prospect have a guarantee from you?
8. Create a situation where the buyer is absolutely not at risk.
9. In 25 years, I’ve never done a mailing with a sweepstakes that was outpulled by the same offer without a sweepstakes.
10. Convert “subscription” offers into “membership” offers — they’ll renew 10% better. People would rather join than subscribe.
11. The bigger the envelope, the better it’ll perform.
12. Unique formats (pop-ups, rub-offs, pull tabs, etc.) always work.
13. Offer installment payments. You’ll see a 15% lift if the product sells for $15 or more.
14. If you’re mailing first class, don’t keep it a secret. Be sure the envelope is clearly printed “First Class.”
15. Personalize. I love seeing my name in print. I love my name. I can’t see it enough.
16. Never write a letter without a PS. It’s the first thing prospects jump to. Use it to restate the benefit.
17. Make it easy to respond by including toll-free numbers, prepaid envelopes, etc.
18. Seventy percent of your words should be five or fewer letters long.
19. Do underline important phrases and words.

Source: A presentation by David Avrick at this year’s DM Days Conference & Expo in New York.