If you have friends who have bought or sold a home recently, ask them if their inspector did a good job, and whether or not they'd hire that person again.
Agents deal with home inspectors every day, so they know who the good ones are.
I have been in business for 21 years and have inspected over 7,300 properties.
Credentialing varies from state to state, so ask about experience, continuing education, and whether or not your inspector has membership in The National Association of Home Inspectors.
Review the written disclaimers that the home inspector will be using in his report as well as the proposed scope of the inspection. Make certain all areas that are important to you (crawl spaces? the roof? drainage?) will be covered.