A very effective plan is to provide property sold statistics on a quarterly basis to your entire list, prospects and past clients. They value this information and you'll get thank-you notes from many of them. Most MLS systems have reporting capability that will produce quarterly sold property numbers. Convert this into a .pdf or other format and attach them to the email or post them on a web site and send a link.
Real experience of one broker with a 1000+ list shows only two or three opt-out requests per quarterly email. That's indicative of an effective prospect retention campaign.
