If you snooze, you lose. Don't assume because you've gotten customers to buy from you once, they'll be customers for life. Even as you read this, your competitors are designing strategies to lure them away. What incentives are you giving clients to keep them coming back?
If you own a coffeehouse, for example, you could implement a "frequent sippers" program that motivates customers to buy nine cups of coffee so they can get the 10th cup free. Any business can celebrate customers' special events such as birthdays or anniversaries by sending them personalized cards offering one of your products or services for free, or at a discount within a limited time.
Rule of thumb: Treat your clients so they feel it's to their advantage to keep their business with you.