The following dos and don'ts are from www.weddingchannel.com. For more great information, check out there site.
1. Don't: Choose a Long Song
Time your song. Even four minutes can seem like an eternity when you're out there on the dance floor with everyone watching. If your hearts are set on a longer ballad, ask your band or DJ about having it cut down to a reasonable length.
2. Do: Consider the Song's Danceability
Face it: There are some songs that just aren't easy to dance to. And there are others that you can't help but move to. Put the song to the test and practice dancing to it in the living room. If the song doesn't keep you two interested and moving for its entire duration, skip it.
3. Don't: Choose a Song Just Because It's Trendy