Megacon doesn't have a skit contest. You -can- do a skit, but you only have a minute, and it won't be judged. Its preferred you just do a walk on and off.
Jacon has not had much demand for skits (last year they allowed for people do to a skit if the wanted, only two out of 100 contestants did one). Chances are it'll remain a costume contest only, like Megacon.
The only cons that have skit contests, are Metro and AFO. Metrocon has it a standard format, with everyone doing a skit, judging, and the awards. AFO has a different system, where you do your skit and the judges will flash numbers as to their rating (like the Olympics style). Its a bit faster and more for the entertainment factor of the judges reactions, than really the skits themselves.