Go to workshops and clinics. In both New York and Los Angeles, there are an overwhelming amount of classes and workshops one can take in order to meet casting directors and agents. The trick is to find a workshop that's worth your money. Mostly what these workshops consist of is a one- or two-day intensive session, where an small group of actors will meet with a casting director or commercial agent, read some copy from commercial scripts and get feedback. If you do a good job, these casting directors and agents will want to work with you. Buy a copy of The Ross Report. This has a listing of every legitimate casting director and agent in every major city in the United States. If the casting director or agent teaching the class isn't listed in there, he isn't legit, and the class will be a waste of your time and money.
Network with other actors on your level. A big part of breaking into any part of the acting community is creating a network. This allows you to check in with a number of people to see what they are up to, how often other people are auditioning, and for what. Often, if you have an agent or manager and you hear about an audition from another actor, you ask to be sent out for that spot if you think it is right for you. Nine times out of ten, the suggestion will be appreciated, and you may even land the role.
Live by your reputation. The acting world is a small one, and what goes around definitely comes around. No one ever wants to work with someone who is difficult. If you are lucky enough to land a job early on, no matter how small, be incredibly accommodating and easy to work with. People will remember this. Directors, agents and casting directors will refuse to work with actors, no matter how talented they are, if their reputation is that of a difficult prima donna.
Take each audition seriously, but not too seriously. Any time you are waiting outside an audition room at a commercial audition, there are always groups of people who are talking too much or providing some kind of unwanted, and occasionally intentional, distraction. Be focused when you arrive. Sign in, read the copy and get yourself as comfortable with it as you can before you enter the room. That being said, have fun! You need to be prepared, but loose. Auditioning for a commercial is a fine line between a Zen kind of relaxation, and an intense focus that will nail the part. Just remember that no matter what you do, or how good you do it, it ultimately isn't up to you. Knock 'em dead, and make them want to call you in for something else.