These have worked wonders in my classroom!
I simply cut them out, laminate them and stick a magnet on the back. Then, every time the class gets off task, too loud, etc, I can take one of the letters down.
If I think the class is doing a really fabulous job, I put one back up!
If the class loses all the letters, they have to owe one recess. I have also allowed them to do something really fun if all the letters stayed on the board throughout the day!