I did a day on “In Plain Sight” (TV) and “Odd Thomas” (film). Generally, it is super tedious work—lots of waiting for the crew to set up the shot and long days. (Also, other folks who have signed up to be extras are generally not my kind of folks, but your experience may differ.) I was luckier with the TV episode because we had kids on set which meant a shorter day by law.
You can count on being fed well, though. The food is almost always very good and there’s plenty of it.
I signed on with On Location Casting. It hasn’t cost me a dime, although there’s an option to pay for premium services or something. Basically, everything is online and on facebook. Your profile is on their website, and when they need talent, they put out a call in their facebook page. If you meet the criteria and are available, you respond to their post and they book you for that day. Next they send instructions about where to go and what to wear. Pretty straightforward all around.