Sometimes I feel like I'm overkilling songs we're learning - having the kids sing them over and over again until they hate the song. (Okay, hate may be a little strong, but I start getting that feeling too like they're dreading having to sing it yet again!)
I've recently discovered that often times it's just that they're singing very softly because they really do know the song well already. So I will just randomly tell them that on the next verse, next time through, or whatever, I'm going to be watching for the person watching me the best and singing (emphasis on actually singing!) the loudest and that person gets to pick their favorite song for us to sing. It's work wonders for me in Senior Primary especially!