These are your client managers. They also may be a client's project manager or project architect. In addition to project work, they are responsible for staying in touch with the client when a job is not in progress. Determine what format is required to maintain the contact. A telephone call may be enough.
Social events can provide an opportunity for staying in touch.
A combination of personal contact, in addition to other communication vehicles, is usually the best course of action. This ensures both that the contacts are made and that follow-up actions are taken. Establish some type of format for reporting and centralize where the reports go when completed. Marketing staff should be fully integrated into the process and may be the best source for making sure that the contact schedule is maintained. Your firm's contact program must be based on the resources at hand.