i cant give you much advice because i only did it once.. it was the easiest 8 hours work i ever did and the best pay i will get for a heck of a long time... if u can pull an agent and dont mind free food and sitting on your ass for 99% of the time then go for it!
The best thing to do is go to an agency but sometimes they just post ads in the paper.
I've managed to score extra parts in a few local movies such as One Perfect Day and The Night They Called It A Day and you get a heap of free food and the pay is pretty good.. the only thing is that it isn't constant work.
my cousin was a extra for heartbreak high ! (one of the schoolkids in the background lol)
he got paid big bucks for it
and all u do it sit on ur ass and wait till people tell u wat to do lol
yeah it was with an agency also
not sure whom though!