PERFORM your skit
This is one of the biggies. No matter how funny your script was, if you stand there delivering it like you've just got out of bed at three in the morning, it won't make anyone laugh.
The same goes for songs. I've seen skits of people singing and they werent entertaining at all. Don't just sing the song, perform it! You have no idea how much a little dance or something can totally enhance your performance. Nobody wants to see you standing and singing. If they just wanted to hear the song, they could listen to it on their CD player, probably sung by someone better than you, too. What makes your performance better than a CD?
Does your skit really need to be that long?
Consider how long your skit is. While some cons have restrictions on length, others dont. For the ones that dont, keep it reasonable. For songs, consider if you really need to sing the whole song, or just part of it. At the last con I went to, a girl sang Melodies of Life from Final Fantasy 9. She wasnt the best singer, she was obviously nervous and she stood there and just sang, no performance or anything. By the end of the first chorus, I think most of the audience wanted to shoot themselves, and we still had to sit through however many verses there were left.