Here's an interesting way to encourage repeat orders. Rather than offering a discount, offer a credit.
An added bonus of the credit idea is you are not reducing the perceived value of your products by reducing the price. Rather, you are selling at full price (and full value) and giving the customer virtual dollars to spend. Sales are boring, and “come back and we’ll give you 10% off something” is overdone and not that exciting.There's even a sample e-mail for you to copy.
Like I said yesterday, maybe all you need is One Good List.