I recently bought some clothes at The Gap. As part of your receipt, they give you a coupon for 20% off your next visit if you go to their feedback website and answer a few questions. The silly thing about this system is that if you say you are a 12 year old girl (or boy), then they don't ask you any questions. How pointless is that? Well, I'm sure that solicitation of young children has a law against it. Tell that to the priests. Heh.
If I was designing the system, I would print out a special code on the receipt that the receipt holder had to enter in. That way you have tracking from purchase to coupon. It sure would tell you a lot more about your customer base than just handing out 20% coupons does.