Every time my students do a written homework assignment I give them a sticker. For M/W/F classes, when they get 30 stickers they get a present (Tu/Th = 20).
They can choose either a pencil or an eraser; most of them go for an eraser, in part because there is such a huge variety of novelty erasers here. For example:
That is a pic of my current stock in my "sticker present" bag. There are Angry Bird erasers, food erasers, and others. For more detail:
The pencils are usually neglected: