I just received the position of the music leader in our ward.  But the idea I'm giving you isn't mine, it's from the one who had the position before me.  She would bring a hat to Primary.  (different ones for different holidays/occasions) etc.  She would watch for which class sang the best and their teacher would have to wear the hat during the next song.  She would continue doing this during all of singing time.  The kids loved this because they would laugh when they saw their teacher in the hat.  It made them want to sing their very best.