Lip sync songs from a popular movie or play could work for a contest. You could lip sync the song like it was sung in the movie, complete with costumes and set pieces that look like the movie. For example, sing "Men In Tights" from the movie "Robin Hood: Men In Tights." This can be especially successful when you have everyone dress up in Robin Hood outfits like those in the movie. You could also get a group together to do "Great Balls of Fire" or "You've Lost That Loving Feeling" from the movie "Top Gun." Again, have those participating dress up as the characters in the movie. Another idea is "You Can't Stop The Beat" from the movie Hairspray. It works because it's upbeat and has a lot of people singing in it.
For a girls-only group, lip sync to "Cell Block Tango" from the play "Chicago," or lip sync song from the play "Wicked."
Oldies songs are another idea because most people in the audience will recognize them and possibly sing along tot he performance. You could choose from artists such as Johnny Cash, Elvis, The Beach Boys and The Beatles. Dress up like these singers and mimic their movements. Other oldies songs that you could lip sync to are "Born To Be Wild," "Blue Suede Shoes," "Tutti Frutti" and "Twist and Shout." Anything that is high energy and well-known can be successful.
Lip sync to more popular or recognizable songs. If you're going to do something like "Thriller," by Michael Jackson, really get into it and dress up like the characters from the music video. Rather than just performing the lip-sync, learn the entire dance from the "Thriller" video with the rest of your group and perform it on stage. Or, lip sync to "Vogue" by Madonna and also learn the dance moves from the music video. Choose a song that's popular around the time you're going to perform the lip sync. For example, if you were doing a lip sync in the late-90s, pick more popular artists from that time such as Britney Spears, Hanson, The Backstreet Boys and Blink 182. Choose something that's fresh in everyone's minds at the time.