Location Matters: If you are close to big cities like Los Angeles, New York or Dallas - it will be much easier to obtain work as a movie extra. In Los Angeles, you can simply register for Central Casting in Burbank and literally be on a movie or television set within a few days. In New York you can find extra castings online or mail your headshot to some of the soaps that shoot in the city. In other areas you will want to contact your state's film commission and ask them who are the "extra's casting companies" in your area. Once you find out you can call or email them and ask to get into their database for talent.
Check Craigslist: Craigslist.org often has extras casting calls in their talent section under "gigs". Be warned that some jobs are unpaid or very low pay, but you can do a few of these jobs to learn what it is like to be in a production. Never pay any money to work as an extra or sign up with an extras casting company.
Join Extras Access: There is a website called extrasaccess.com that lists various extra roles throughout the nation. This is a great resource for extras or background talent looking for work.
Call Local Colleges: If all else fails, you can call your local colleges and ask if they have a film program. You can inquire with them and find out if any of the student projects have extra roles or maybe even small speaking parts. This is a great way to gain experience, but don't expect to get paid!