In working with the client, you have invested a lot of time creating a perception that can result in referrals. If you don't maintain the relationship, you will lose the benefit of the good perception you have established. For a client to make a referral, it is necessary that there is a perception of a continuing relationship in the mind of the client. As time passes without any contact, that perception is lost. It is important that you remind your inactive clients periodically that the relationship continues.
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.