No formal program. I am sure to thank them and take good care of them on future business. If it's a corporate account that leads to a fair size job / installation then surely a nice bottle of wine is warranted.
I don't have a formal program for this either. I make sure to thank them and expressing how grateful I am. They know I freely give my time and attention to whatever and whomever comes through my doors. They recommend me for a reason. I do not offer coupons or discounts. In fact, quite a few of my customers would be embarrassed if I did.