Good customer service is invaluable and can be as simple as: answering your phone, doing rush jobs, or following up with clients.
When was the last time you called your client to see how they are doing? When was the last time you checked with previous clients if they need anything done? You would be surprised just how important constant, friendly, unobtrusive communication can be.
Be there for your client, and they will be there for you. Check your spelling, grammar and clarity of your messages -- voice or electronic. Be pleasant to deal with and it will come back to you many times over.