Take commercial acting and audition classes. Research online or contact a local casting agent for a list of professional instructors. From full semesters to one-day workshops, useful classes may include everything from how to create a character to script reading to audition skills.
Get your head shot taken by a professional photographer. It should be a natural shot that looks like you right now. That means no beard if you're now clean shaven, no long hair if yours is now short. Get copies in size 8 by 10 and on postcards to mail out to potential agents.
Write your acting resume, or hire someone to prepare it for you. Include all training, education, experience and special skills. Special skills should list your unique abilities, everything from speaking a foreign language to juggling to riding a unicycle.
Seek out and obtain a commercial agent. This will put you ahead of the game in many smaller markets and is a necessity in larger markets. Certain casting directors will only cast commercials through the use of an agent. They will not post auditions directly to the public. You can call local agencies to see if they accept walk-ins. Email and snail mail your head shot and resume. Be persistent but respectful.
Stay involved in the acting community. Register with online forums where local actors meet and share news. Frequent message boards where auditions are announced. Getting your face seen and your name out there in the community really is important. You never know when the person you meet today might give you a job tomorrow.