I use this rewards system to manage the behaviour of the class. I divide the class into three teams (ghost, zombie and vampire), and place the characters on the whiteboard. If the team answer a question correctly / are behaving nicely they get one point, if they are misbehaving they lose points. At the end of the lesson, the team with the most points can colour a banana. At the end of 9 lessons I reward the winning team with stickers, candy or certificates.
This has worked wonders with my classroom management and has taught the children about delayed gratification as they have to work towards the prize over a long period of time.