How do I become an extra?

I have no real desires to be an actor, I would, however, love to do some extra work. I think it would be fun and would be a cool side job to make a little extra cash. I’m in my mind 20’s and of average height and not bad looking and I live in NYC so I imagine there are plenty of opportunities. My problem is that there are a lot of scams round these parts that are out to take advantage of young naive actors coming to the city. On top of that I just don’t have a clue on how to enter this industry. So, where do I get started?

Also, if anyone has experiences with being an extra I would love to hear those, good or bad. Especially if you have specifics about agencies or what have you in NYC.