The measure of success is not whether you have a tough problem to deal with, but whether its the same problem you had last year. (John Foster Dulles)
It generally costs six times more to win a new customer than it does to get an existing customer to purchase again. Like all general "rules", this assumes certain things - such as the customer hasn't been so put off by their experience they never want to do business with you again.
Strategies to get them to return include:
Classifying customers. Tiers allow you develop appropriate marketing for each group, rather than a blanket approach for all. A common example is:
Asking them to return. As simple as it sounds - if you've already delighted them.
Providing awesome service. Again this sounds simple - but it means consistently providing service that goes above and beyond the norm. If you have a team, that means ensuring everyone is committed to and trained for awesome service.
Nurturing customers. Think of this as a similar to a marriage. It requires effort and attention and communication - real rather than pre-packaged. The more customers you have the harder this is. Personally I pay no attention to such messages from my power or phone company. They don't know me at all (except as a customer). You could develop an annual communication calendar, send regular mailings, establish a loyalty programme and make follow-up calls to ensure 100% satisfaction.
Using customer comments. After a purchase this can ensure satisfaction and avoid post-purchase dissonance.. You can also use random phone calls, satisfaction surveys, web-based feedback and customer advisory boards.
As with other challenges, the key is always to break the process into manageable steps systematically.