The formula is simple:

First, pick a goal or an accomplishment (saving money, increasing performance, improving health, avoiding pain, etc.). Then tell the reader how to accomplish this goal in X number of steps. Or give them X number of ways to achieve the same goal.

Here are seven tips for using numbered headlines on your direct mail postcards:

1. Use odd numbers.

With all other things being equal, odd numbers have been shown to outperform rounded, even numbers. They appear more legitimate and scientific. That's why Listerine "kills 99% of germs" instead of 100%.

2. Write numbers as numbers.

Numbers catch the eye better in numerical form (7), as opposed to written form (seven).

3. Crank up the benefits.

To increase the motivating power of your numbered headline, simply increase the reward promised by the headline. For example, instead of "7 Ways to Decrease Your Debt," you might say "7 Ways to Eliminate Your Debt." Only one word has changed, but now the promise of value is much greater. "Decrease" is vague. "Eliminate" is absolute.

4. Write clearly.

Keep your headline simple and straightforward. The reader should understand the benefit right away. She should not have to read the headline twice or puzzle over it. Number and benefit … it doesn't need to be any more complicated than that.

5. Use strong action words.

Instead of telling readers how to "increase the safety of their computer systems," tell them how to "safeguard their data and shield their servers." Strong words conjure up strong emotions. Boring words yield boring results -- drop them from your direct mail vocabulary.

6. Deliver on your promise.

If your postcard headline mentions "21 Ways to Increase Your Email Marketing Success," you better follow up with (you guessed it) 21 ways to improve the reader's email marketing. At the least, offer a summary or excerpt of those tips and then point to a full report that offers the rest.

Stay away from "bait and switch" headlines that don't deliver on the promise. Trickery and deceit will get you nowhere.

7. Follow a formula.

Develop a formula when creating your headlines. This will help you ask all the right questions and shape a headline around the answers to those questions.

Example formula:

Audience
Product
Number of parts
Benefit
Headline

Now let's imagine we're a financial services company using this headline formula for a direct mail postcard. It might go something like this:

Audience: Middle-income taxpayers
Product: Financial seminar
Number of parts: Three
Benefit: Saving money
Headline: Get More Money at Tax Time -- 3 Simple Steps

You can see how the formula builds the headline. It also helps us identify the heart of our message and the audience it's intended to reach.

Bonus Tip: Write a Dozen, Keep the Best.

Headlines can make or break a direct mail postcard. So don't settle for your first attempt. Even professional copywriters rarely hit the mark on the first try.

Personally, I'll write at least a dozen versions of a direct mail headline. Then I'll eliminate half of them (the weaker half) right off the bat. Then I'll put them aside for a day or so, and when I review them again I'll pick what I think is the best one.