Start your search by asking some writerly friends for references. Ask with whom they're working and if they're satisfied.
Find books similar to ones you want to write and make note of the name of the literary agent who represented the author (they usually get thanked in the acknowledgments).
Visit agency Web sites for information on genres they specialize in, clients they represent, preferred method of communication, and any guidelines for submissions. Check if you can send in a full manuscript. Most agents don't read unsolicited manuscripts and many won't read unsolicited proposals.
Consider both large and small agencies. The bigger ones may have greater resources; the smaller operations may give you more personalized service. If you're taken on by a larger firm, ask who will be your agent; the agency may have a marquee name, but a newcomer, junior associate or assistant may actually handle your account.
Have any agent outline exactly what he or she will do for you, and what the specialty is. Some agents will help you land magazine assignments--others will work only on book projects.
Find out how long the prospect has actually worked as an agent. If the person is relatively new to the field, ask what he or she did before, to gauge what publishing connections can be brought to the table. An agent's greatest asset is his or her relationships with editors.
Get a sense of how aggressive an agent might be. Some will go to the ends of the earth to sell your project. Some won't go any farther than the end of their desks. Ask to talk to past clients to find out which category this agent falls into.
Ask about commissions. Most agents charge a 15 percent commission on anything they sell on your behalf, and some charge for incidentals like photocopies and postage. And since agents only earn a fee when they have generated income for you, they have a strong incentive to work on your behalf.
Request a written contract that confirms everything you and your new agent have agreed to.