I use Bonus Bucks as a reward system. Basically, all you need to do is make up some "bucks," copy them, and then cut them out.

Whenever a child does something good (you define what "good" is), the child gets a bonus buck.

At the end of a term or whenever you wish, have prizes for the children to "buy" with their bonus bucks.

Give each prize a value and then set up a store or auction for the students to purchase their prizes. It is a great motivational tool and a lot of fun, too!