Do you sell products that lend themselves to repeat purchases? This could be vitamins, pharmacy, contact lenses, hair product, office supplies, grocery or anything that you expect someone to "use up."
Amazon sells, amongst other things, coffee beans. Check out the offer for free shipping and a 15% discount for customers who want to subscribe to the product:
Here's what Amazon's doing right:
1. Offers of free shipping and discount are strong motivators for repeat purchases.
2. Allows you to select the subscription schedule for 1, 2, 3 or 6 months.
3. Provides a customer service, no need to return to the site again, place order and wait when quantities get low.
4. The offer is placed in the product description and right near the cart button. Impossible to miss if you want to buy this product.
I bet you'll be hard pressed to find many other online retailers taking advantage of this technique.