Don't forget to ask for references from several investors who purchased deals with them.
Also check out some of the REI clubs in your area (or in the areas where you plan to invest), and network with some of the agents that show up there. You need to make sure that you're working with someone who understands what you're trying to do. Most agents aren't trained to work with investors, and they don't understand how to analyze deals the way an investor would. I agree wholeheartedly with Darin: the best agents to work with are ones who also invest.