Lip-synching skits are pretty much a no-no. They get boring and very awkward. I would recommend writing a skit with your own dialogue. It's more work, but the audience and judges will appreciate it a lot more.
Finished: Armin Arlert, Mayoi Hachikuji, Tomoko Kuroki, Ultimate Madoka, fem!Masky
In progress: Kougyoku Ren, Lightning Farron
Upcoming cons: Sabakon, Youmacon, Con+Alt+Delete