i live in LA, where - LITERALLY - every single person you meet is an extra trying to become the next big star, so, my answer is bound to be very different, and probably not very helpful. If you lived here, it would be SUPER easy to be an extra in something. They put fliers on telephone poles for extras, there's casting newspapers in every corner store, or chances are someone you speak to during the day is making a movie and could use extras.
there are a few agencies that deal only with extras casting, however, and if you go through these places, it could almost become a steady (low-paying) job. They require you to come in and sign up, and they take a list of your skills (playing an instrument, contortionist, alcoholic, etc) and a polaroid of you, and then they call you the night before they think they could use you for something. they tell you where to show up and what kind of clothes to bring ( "today you'll be a skater punk, so dress like that", or "today you'll be the big lumberjack sitting at the end of the bar"), and you just do it. most of your day is spent hanging around with the other extras and eating craft service (snacks). then, you'll probably have an hour or so of work, where everyone is rushing around all panic-y, and at night you get to go home and they tell you if they need you the next day or not. I think they pay something like $50 a day, maybe some of the better ones pay $100.
the real gold is struck when they pick you out of the crowd. Sometimes they'll ask "did anyone here drive here in a VW beetle?" or "Is someone here 6 feet tall with red hair?". They'll be looking for something specific. If you fit the criteria, your pay just went up to maybe $300 for the day, and you may be whats called a "featured extra", or maybe they need you as a stand-in for one of the big stars.
And if they need an extra to deliver a line on camera, then you REALLY make some good money for the day! but that's rare, i think...
these would be the first places I would look, but where you are, I have no clue. Maybe one of these places have links to your area or something.
http://www.backstage.com/bso/index.jsp
http://www.centralcasting.org/index_intro.asp
http://www.la411.com/Production_Support ... /Index.cfm