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.
Fandom Fest 2014 cosplay: GLaDOS (Portal), Ellie (The Last of Us), Lara Croft (Tomb Raider 2013)
MCM London Comic Expo cosplay: Oktavia von Seckendorff (Madoka Magica), Fem!Slenderman,
Future cosplay: Faith Connors (Mirror's Edge), Lara Croft (Rise of the Tomb Raider), Tetra (The Legend of Zelda: Wind Waker)
2015 conventions: Anime Los Angeles, Anime Central, Japan Expo