Live in an area with a suitable market. If you want steady work, this will almost certainly mean the Los Angeles area, or possibly New York. And be aware that "area" means a very big area. Gone are the days when "Hollywood" literally meant Hollywood. "Hollywood" studios are now located in Hollywood, Burbank and Culver City, just to name a few places. The actual shooting, in any case, generally does not take place at the studios, but at some location within a radius of 50 miles or so.
Have reliable transportation. You just can't depend on public transit, particularly in a place as spread out as L.A., to get you to and from your location quickly enough. Period. (And in some cases, there may even be an opportunity for your vehicle to appear on screen. Maybe your car can become a star.) A GPS would be a very wise investment as well since you'll probably be traveling to some unfamiliar places.
Have a varied wardrobe. Studios have their own abundant supply of wardrobe, of course, and you'll most likely wear some of it---especially if you're portraying a distinctive character such as a police officer. But in some cases, you'll be asked to bring some items with you. And if they like what you bring, you're more likely to end up closer to the camera.
Special talents help. If you can juggle, do contortion or play basketball, those skills might get you assignments you wouldn't otherwise get. Moreover, they could get you high-profile assignments that might qualify you for membership in SAG (the Screen Actors Guild, the film and TV actors' union). The Steven Seagal film "Fire Down Below" had scenes set in a casino, so casino dealers from the Los Angeles area were brought in to be extras, even though most of them had never worked in film before. And after three days of shooting, they'd earned a SAG card---an achievement for which some extras struggle for many years!
Have a headshot. This is not a necessity, as it would be if you were auditioning for speaking roles; the casting agencies can take their own photos of you. But if you have a professional headshot they can put on file, it might help "sell" them on you when they're looking for a particular type.
Register with one or more of the casting agencies for extras. The biggest one, the one you've heard about for years, is Central Casting in Burbank (and New York). But don't overlook the other good agencies as well: Axium, for instance. The more reliable agencies you register with, the greater your chances of working steadily.
Be persistent. Call in at the designated times to seek work, and keep calling until you succeed.