Let's face it: we all judge books by their covers. And we judge the direct mail we receive by the envelope. In this column we'll examine why some envelopes get opened.

Guerrillas are familiar with the two basic approaches to envelopes. One is a simple, elegant white #10 envelope. In "Guerrilla Marketing for the 90s," Jay Levinson states that the standard white envelope with a first class stamp and no return address is intriguing and results in high response rates. When using color, blue and red are the traditional choices. Levinson also suggests using vivid colors like yellow, orange or pink.

The choice of color should be appropriate to the product being offered. If you're marketing medical services or life insurance, stick with neutral colors. However, if you're offering computer software or sports equipment, go crazy with color.

Your envelope can offer a peek inside through a glassine window to a colorful photograph of your product, a membership card or contest entry form.

If you have a product or service that's fun, think about enhancing your envelope with rubber stamps or post-it notes. Rubber stamps printed in red or black suggest urgency. Be sure your rubber stamp effects look authentic: a border with rounded edges helps. Design the rubber stamp effect to convince the recipient that you added this stamp just as the envelope was going out.

Obviously you can't actually put a real post-it on the envelope as it would never survive the postal journey. You can print a post-it note effect. Be sure to add a drop-shadow. If these are done well, the recipient may even reach out to detach the note.

Now, what should you say on the rubber stamp or the post-it note? Some marketers have found success by turning their recipients into co-conspirators. You can accomplish this with a "they don't want you to know this" warning. ClichŽs though they are, "Private Invitation" or "Free issue! Better grab it fast!" or "It's our 50th anniversary and you'll celebrate it with us" might get more envelopes opened.

Some Do's and Don'ts: Don't promise the moon unless it's actually in that envelope. A wild promise, with no payoff, results in bad faith. Do offer the opportunity to win something. Contests are perennial favorites. Do use the word "free." We all love getting something for nothing.

Remember, your envelope is a silent salesperson. Does it generate a desire to buy, where the desire didn't exist before? Does it transform someone who is just looking into a buyer? You can help get that envelope opened with the right choice of words. Guerrillas know that power phrases include:

* At last!
* Finally
* Now you can
* Exclusive invitation
* Free gift
* Enclosed you'll find...
* Congratulations
* Private information for addressee only

Remember, the only purpose of the carrier envelope, other than keeping its contents from spilling out onto the floor, is to get itself opened. Follow these tips and watch your response rate grow!