One of the biggest benefits of direct-response marketing (whether via snail mail or e-mail) is that you can get almost immediate feedback from your market. You should constantly be testing to see if you can beat your “control” mailing.

So what should you be testing? Obviously you should always test your lists. Today, you can refine and redefine your target audience to the nth degree. Don’t be afraid to segment and sub-segment.

You should also test your offers. Does a certain dollar amount discount create more inquiries than a 20 percent off deal? What if you add in something free rather than taking money off? How about putting in or shortening a deadline? What does that do to responses?

Those are the basics. But where do you go from there?

Copy length: Many direct-response writers swear by the maxim “the more you tell, the more you sell.” Though some still argue that’s true, others point to the USA Today mentality of today’s audiences. They want it fast, visual or with bullet points.

Headlines: This is the make-or-break moment. The headline either lures readers in or tells them not to bother going any further. Your headline should be the “once upon a time” part of the story. It sets the stage and tells readers what’s in it for them.

The P.S.: This is the part most people mess up. They don’t include one. In readership studies, the P.S. is one of the most read elements of any direct-response piece. In many studies, a piece with a P.S. outperforms 3-to-1 the same piece without the P.S.

It’s a rare marketing tool that can be so specifically tracked, measured and analyzed. Don’t miss the opportunities that it can afford you.