People tend to associate with others like themselves. The rule here is very simple. Stay in touch with the clients you would like to serve again. Staying in touch with clients who were difficult or couldn't pay their bills, is asking for more of the same. Any time there is any change in your business you should send out an announcement.
Most attorneys send announcements when they start a new firm, go solo or change addresses.
This usually brings a flood of calls wishing them well. It also brings some new matters. Expand this idea to include less monumental changes. You don't have to print up engraved cards to send an announcement. For minor announcements, a form letter written on your word processor is sufficient.