If I were filling in for an unknown band, I would require one-half of my pay upon arriving at the gig, with the remainder to be paid at the end of the gig. I have known several musicians who have been stiffed on these types of gigs due to "unforseen expenses" or "the band got stiffed by the club owner" or even "dude, you've owe us for the PA rental."
Quite honestly, I find that most bands that post in craigslist are full of it. If they tell you they have a paid gig, it probably means that they are playing for tips. If they tell you they have a good following and they are playing for the door, that means that their girlfriends will show, but nobody else. Never agree to take a % of the door or a split of tips. Get your amount negotiated up front.
Another common practice, especially in music towns like Nashville, is for struggling "artists" to try to con unsuspecting musicians into helping them record a demo for free, with the promise of paying gigs "once the demo is cut." I had a number of these types contact me when I was looking for a band.