How to maximize after sales opportunities
If you running an e-commerce portal and would like to leverage on existing users by getting repeat orders, the essence is to treat them well after they placed their initial order then only there is high probability of repeat purchases.

Maximize Cross Sell opportunities - Cross selling is the strategy of pushing new products to current customers based on their past purchases. Cross selling is achieved by including in your confirmation email other items they may like, as well as mailers included when the product is sent.
The essence of any successful cross-selling strategy is recommending the right product at the right time. To do this, you must have information on the purchase history of the customer and product relationships. Cross-sell items in similar price points, or higher priced items
Incentive customers to buy again – Always include coupons with the package. Customers who have just brought from you are your best prospects to come back and buy again.
Consider product and money return policy - Having a money back return policy shows customers that you stand by your product and increases their confidence in buying from you. It also shows that you care about the satisfaction of your customer. Specify clearly how the customers should handle the process of returning the item, and when and how they can actually expect to get their money back. However, use this feature only if you can back it up and you are indeed willing to return their money back.
Unless you are selling questionable product, few people will actually take you up on the offer.
-Hitesh, vcBytes.com