Agents want experience. Get involved in a play or showcase and send invitations to your targeted agents. Check the trades for casting calls, and contact casting directors directly. Always keep your resume current and remember to include all union affiliations. Submit appropriate photos.
Commercial agents require different types of photos from theatrical agents.
If you have a film or tape of yourself you may want to submit it either in addition to or in lieu of a photo. An audio demo should generally not exceed three minutes. Always send your submission to a specific person at the agency. Indicate in your cover letter that you are seeking representation and state why you would like to be represented by them. Keep notes of the agents to whom you submitted your pictures, as well as the date of submission and any response or comments.