Treat your customers with respect. Acknowledge them and go the extra mile all the time. Free picture hooks. Helping them carry a large picture to their car. If you don't have it, find out how to get it for the customer.
Courtesy calls on back orders.
Don't call them and you'll hear the rage when the job isn't ready on time. A clean store. Free delivery if the art is too big for their car. To Do Business With You. All of the above actions and many more just like them will bring you all the referrals in the world.