Direct marketing is an individually targeted marketing tool, so make the most of the opportunity it provides to talk one to one.
With advances in digital printing technology it is very quick and simple to, at the very least, produce customised sales letters for each recipient, perhaps mentioning their previous history with you or their industry in the content by using the same technique as in a mail merge.
With a little more forethought you can even customise mailers or catalogues with different products, images or copy targeted to different sub groups in your mailing.