Set yourself up as an expert in your field and you'll make an easier sale. How do you set yourself up as an expert when your only contact is through the mail? I'm going to assume, for this article, that you don't have the time -- or the inclination -- to produce a regular newsletter. I'll also assume your printing budget is low.
Several years ago a client opened a dry cleaning shop and wanted to promote his services by mailing a large (5.25" x 8.5") postcard. In addition to the usual 2-for-1 Tuesday offers, we blocked off a small section of the card and wrote a couple of paragraphs explaining why clothes should be dry cleaned and how it extends their life. On the next postcard (this was a series of mailings to the nearby neighborhood), we talked about how to get difficult stains out.
Worked like a charm. People were calling left and right to get the dry cleaner's advice. And of course more than 3/4 of all callers brought their clothes in to the shop because they felt that the owner knew what he was talking about. He's still in business and still doing well. This method also worked well for a delicatessen that always included an article about the benefits of eating properly (and subtly tied the recommendations to some of the specials they were offering). These two companies set themselves up as (a) experts in their fields and (b) friends who want to help their prospects.