How I did it: I joined three local talent agencies. They contact me whenever there are opportunities and castings available. The agency will tell you what the producers are looking for,how you should look when you go to the casting, how much they're paying per day. On my experience it's easier getting hired for a movie than getting hired to work as an extra in a commercial or music video. The best of all it's getting to see the main characters so close and getting to know people. I saw Benicio del Toro every day and Johhny Depp sat next to me during a break. Besides the paycheck is good.
Lessons & tips: Follow the given instructions carefully. I saw extras playing "smart" being fired for taking pictures to the stars on the set or because they are smiling on a scene that requires you to be looking sad and stuff like that.
Take something to read while you wait for your call.
Resources: White pages to check out agencies.