If you sell high-volume, low-value items (hot drinks, snack food), give customers a loyalty card to be stamped with every purchase. Once they reach the target number, they qualify for a free item. The cards encourage your customers to return to your shop by promising deal for multiple purchases. The more they shop with you, the more they will receive in return.
Tips: Use different rewards for different tiers, this will encourage your loyal customers to achieve the highest tier.