Run a points program behind the scene. You can keep a running total of points based on values you assign to your products.
Set milestones for your customers to reach and reward them when they get there. Or keep it quiet, watch and reward those loyal customers for being just that, loyal.
While a sandwich store is offering the buy 10 get 1 free and redeemable coupons, it is also offering to giveaway a fully catered event when users hit x number of loyalty points. Long term incentive for customers is never a bad thing.