You need to know on which people to spend the most time, energy, effort, and dollars. You might be surprised who is your leading salesperson in the field. You could even have a contest and create a recognition reward for the most valuable past client, a MVPC award. People love recognition from others. If done right you could create a feeding frenzy of past client referrals.
Personal contact is the key.
You need to personally call your client base with regularity. Calling past clients also might make your day, because they will often say how thankful they are for your good service.