Live in or near a city where movies are frequently made. If you live in Los Angeles or New York City, you are much more likely to find opportunities to work as an extra than you are in a smaller city nowhere near a movie studio.
Check for movie extra opportunities on Craigslist, in classified newspaper ads and on websites such as auditions.com and entertainmentcareers.net. Subscribe to these websites to get regular email updates regarding new opportunities and check them daily.
Submit your information to local casting agencies. Casting agencies often require head shots, previous experience and even audition tapes from people who would like to be cast as a large role or even just as an extra. If your information is already out there, it is likely that they will call you in if someone is needed.
Gain experience by taking acting classes. Brush up on your skills, even if you only take some basic courses to learn about the industry. The more you know, the more it will help you find a job in the long run.
Get an agent. If you are serious about finding consistent work in acting, even as just an extra, an agent will be a big help. They understand the industry more than the average person and will be able to find a variety of jobs that are best suited for you and your skills.