The majority of my work is subbing and my experience lines up almost exactly with musicalmason. Luckily, I've only been shorted once (I was hired to play 2nd in a big band, only to have the lead player not show, so I read the lead book instead) where the leader gave me less than he initially promised due to "less of a crowd than they anticipated". I reminded him that I covered his rear end by playing lead instead of 2nd and it flew right past him. Oh well. Once in 10 years isn't bad!
My methodology is: Show up early, be friendly but don't talk too much, play your butt off (treating it like an audition is excellent advice), be flexible, help with setup and tear-down, and be sure to thank the bandleader for the gig. Many bandleaders appreciate and notice if you're one of the first to show up and last to leave.