I have an idea that really works for my primary. Specially with the juniors which can be really the problem sometimes.  I bring in 2 Mason Jars filled to 3/4 with water. (I make sure I place them on a table with a placemat under them), I tell the kids that we are going to try to change colors of the water and the better they sing the darker the color will get. I then start adding food coloring drops into the water of the first jar. little at a time. Once it gets dark (and believe me, they are singing great at this time), I tell them I will change the color to a different one. (If I had dark blue, I would add red and make purple, or if I had dark green, I would add yellow and get blue, you get the picture). Toward the end I tell them that the best singing would really come if we could turn the water white all over again.  If they do even better (they always do), I put some drops of bleach into the water, and then VOILA!!!! The water is clear.  They love the game. it gets the Sunbeams involved and I get them singing well for weeks after this. I don't use it all the time, but every time I use it works like a pro!  Make sure you use a wooden spoon for the mixing, and for better results, get the liquid food coloring, and not the paste kind.  The other jar with water is to be used for another song, as once you added the bleach, the water is no good for coloring anymore. (I know you knew that!!!!!)