As veteran talent manager John Schultz points out, do not expect miracles from a casting representative. Your representative's job is to submit your credentials for the parts you are most likely to win. If you do not have the credentials for a large part, or are in the very beginning stages of your career, you may wish to settle for parts as an extra. Remember, it's the casting director's job to cast the individuals they think will perform the best in those roles.
Having the correct agent and manager is important. As Schultz points out, you need both. An agent may negotiate contracts with unions on your behalf, which is something a manager is incapable of doing. Still, having a manager is important, according to Schultz, because a manager is in the position to work with you individually, answering any questions related to the business you might ask, whereas an agent may not have time to work with you on that basis.
As Lathem points out, training is important. Join an acting class. In these classes, you learn proper technique, including proper protocols to use at an audition. The teachers will not only teach you how to read and memorize lines, but how to be more convincing in your role, how to inject the correct amount of emotion into your role. The better you become at that, the more likely you are to convince a casting director to try you in a role.
You are competing against many others who have acting aspirations as strong as yours. As Latham points out, as with any prospective employer, it is best that you bring a portfolio to the table. Start with two eight-by-ten head shots of yourself. Make sure you have a serious picture of yourself, as well as one with you smiling. These are for drama and commercials, respectively.
In addition, Latham recommends that you write an honest résumé. Your résumé should include any acting you've done. Be honest; if you have not acted in major roles, be sure to include work you've done in a school production, or any training you have completed in an acting school.