I wondered about this subject myself. I've only done two cosplay skits in my time with this hobby... but here's something I've noticed as both a viewer in the crowd and a participant in a skit:
1) HAVE A BACK-UP PLAN. I'd be here all day explaining past cosplay skits where there was Audio/Visual problems, missing participants, costumes falling apart, etc. You can't predict if the skit turns out any good, but it's always good to have a contingency plan. A/V problems seem to be the most reoccuring issue.
2) SINGING. Yeegods, I've heard some bad
ones. I'm not Simon from "American Idol" but seriously folks; if your singing is questionable at best, you have trouble remembering the lines, or if you're in the voice-cracking puberty stage ... just have who ever's working A/V play the tune for you. Or better yet, save it for Karaoke like a lot of cons prefer.
3) APPEASING THE AUDIENCE. Obviously, the skit's got to please the audience and judges. Cracking a joke only a handful of friends might get won't help. Acting out a scene from an anime/game the majority haven't heard of definitely won't help. If possible, work an 'introduction' into the skit so everyone can enjoy it. I wouldn't try pulling people from the audience either, since that's asking for "random" comments and acts from the person, possibly ruining the skit.
4) SKIT STYLE. By far this is the hardest thing for a group to come up with, especially if it's people you've met that very day. Since it's obvious there's little-to-no time to practice... keep it short, sweet, and memorable. Maybe do a satire of the anime/game/genre you're portraying. Or a scene introducing the audience into the cosplay. I don't know about all the conventions, but most of them favor "humorous" skits as an end result as Hikaru suggested. FYI: Longer skits means that's a LONGER time you have to keep the crowd into it.
5) THE STAGE. Definitely check the stage first to see how much room you have, and if needbe compensate for it. So if you notice the stage is nothing more than covered tables... it wouldn't be wise to do anything that'd put a Jackie Chan movie to shame with all the stunts. If you notice it's smaller than planned, change up the skit to either have them exit stage or wait by adjacent to it til' their part comes up.