Contact customers who have not had billing within the appropriate time period for the type service. These are accounts that you have had and possible lost. Try several different programs before checking to "Exclude from mailing". Avoid price discounts, use coupons and emphasize the benefits of your service.
Send service survey to existing customers with information about their account and asking for ways to improve service. Excellent blocking tool against being bumped from the job.
Remember, by contacting an existing customer you keep them as an existing customer.