Quality control is your responsibility. Quality control and good service are part of the same thing. Consistently successful independent restaurants always have an owner on the scene, to ensure that everything is to their customers' satisfaction. Learn from them. Contact, contact, contact.
People have short memories.
You can gently, constantly remind your customers that you still value their business, without becoming a pain in the butt. Letters, flyers, postcards, birthday cards. Whatever it takes. Good techniques, poor execution. In your personal hands, they may work beautifully.