Go to your local theater arts bookstore and buy a directory of the local talent agents. In L.A., for example, you could go to Samuel French; you can also subscribe to the Ross Reports.
Look through the descriptions of the specialties that agents cover, such as children, models, character actors, leading women or leading men. Target the agents who fit your type.
Find out if any of your friends or contacts know the agents you're targeting or are represented by them. Ask about their experiences, and ask if they can give you a reference.
Submit your headshot and resume to the agent, with a brief cover letter introducing yourself.
Follow up with a phone call and a postcard.
Send invitations to all of your performances to agents whom you have targeted.
Perform in a "showcase" (see glossary), but select carefully. Many showcases involve a fee, so find out in advance which agents will attend.
Be persistent but not obnoxious. Send postcards every two to four months updating the agents on your activities.
Prepare a monologue. When an agent calls you into her office, you will probably be asked to do a monologue or cold reading. Dress in a flattering but professional manner, and bring headshots and resumes.
Thank agents for interviews, and if they don't choose you this time around, keep in touch and don't take it personally; they can only take so many of each type of actor at a time.