Keep the shop/lobby/parking area clean, and keep people that aren't customers/promoting the business out. Also, ask people who have their cars in for extensive work if you can display pictures of the work being done in the lobby, and if possible have a shop-sponsored/project car to display how awesome your shop is at building up a car.

I guess other than that I don't have much to add, other than if people come in for fluid changes don't hassle them if they bring their own fluids. Good luck, and I hope you succeed because I need a shop that I trust to bring my Subie to.