every time a first time customer places an order I ask how they found out about us, ie: internet, phonebook, walkin ect. so I can track my different advertising campaigns.
If they say "Pam Johnson told me about you" I send Pam a $50 gift certificate with a quick note thanking her for trusting us enough to refer her friends.
Some time ago I send out a postcard to all my clients letting them know about the beginning of this new referal program, I doubt that any of them even read it, but once they send someone in and I send them the $50 certificate they catch on real quick. So far it has not been a huge success, but I do have a few customers are doing a great job telling their friends about me and getting $50 each time.
The only restrictions: the new customer must place a custom framing order that includes the purchase of a custom frame. and I dont count the spouse of an existing cutomer as a new customer (or anyone living in the same household) but other than that its fair game.