Read your owner's manual to become familiar with your vehicle and follow the manufacturer's suggested service schedule. If possible, arrange for alternate transportation in advance so you will not feel forced to choose a facility solely on the basis of location. Professionally run establishments will have a courteous, helpful staff. The service writer should be willing to answer all of your questions. Feel free to ask for the names of a few customers.
Some facilities specialize.
Ask if the shop usually does your type of repair, especially if you need major work. The backbone of any shop is the competence of the technicians. Reward good service with repeat business. It is mutually beneficial to you and the shop owner to establish a relationship. If the service was not all you expected, don't rush to another shop.