It works with family and friends. And it works with customers. It's a pretty simple concept - do unto others (keep in touch) as you would like them to do to you.
Question: do you communicate with your customers only when they are buying from you or you are selling to them? If so, perhaps you should consider extending that communication to non-selling occasions. Let them know you appreciate them. Thank them. Share with them. Your customers will naturally be more loyal when they feel that they matter to you, that you care about nurturing the relationship.