Prospect mail programs need to build a reputation for themselves to be effective. I generaly recommend a series of three mailings when prospecting via direct mail. Below are my reasons. Let's first assume, however, that you've designed an effective letter and/or brochure.
Mailing #1. Some prospects will toss your mail automatically. Most, however, will read your envelope and make their "open or toss" decision based on your name/logo and envelope appearance. Response, depending on your package and presentation, and with exceptions to every rule, will probably be small.
Mailing #2. (about three to six weeks later) Some prospects will again automatically toss your mail. The rest will read the envelope and remember your name from somewhere. More people will read your offer. Response will generally increase over the first mailing. You're beginning to gain your prospects' trust.
Mailing #3. (about three to six weeks later) Well, some people are die-hards and will automatically toss your mail no matter you do. The rest, though, will remember your name. They also realize by now that you're not a fly-by-night company and that you want their business. By the third mailing you've gained your prospects' respect and your response should greatly increase.
Additional mailings. You have to use your professional judgment past the third mailing. Whenever possible, weed out the long-shot non-responders. The more often you mail to your best prospects, the better your overall response will be, but there is a point of diminishing return. If possible, consider implementing a telemarketing program where you try to reach your prospects personally.