Want a better response to your envelope mailings? Try using a "lift" label. This device gets its name because it has a tendency to increase or "lift" your overall number of responses.
The traditional use of this technique is with a "lift letter" usually folded in half and inserted into the mailing itself. It usually says, If you've decided that this product/service is not for you, read this important message. The inside message generally includes a testimonial urging you to try the product or service.
A lift label, on the other hand, puts a message right on the envelope itself. It can say something like ... Free sample enclosed or Early registrants to the XYZ conference will get free tickets to Baltimore Orioles games or whathever works for you. The lift idea is to give the recipient a darn good incentive to open that envelope.
Make sure the message is noticed: Print your message on a colored label (red, green, yellow, blue, etc.) that clashes with your envelope color. Your message will stand out better.