From Polka Pics
The original source suggests that parents add a cotton ball or a marble every time a they catch their child being good, but it is really easy to adapt this for classroom use.
Any time your whole class is on task, being good, someone goes above and beyond, etc. add a cotton ball or a marble to the jar. When the jar is full, the whole class gets a reward. What is that reward? You and your class should decide when you start so they know what they are working toward.
Any time you catch a student being good, write their name on a piece of paper and add it to the jar. Then, when you need someone to make a copy, pass out papers, or help in any sort of way pull out a name.
Rather than having 15 eager volunteers turn into one excited student and 14 disappointed ones whenever you need a helper, students will now know that their ability to be your special helper is determined by their positive behavior (and the luck of the draw).
If you use option two, make sure that you catch every student being good on a somewhat regular basis. You don't want a student feeling like she can never do anything "good enough" to get her name in the jar.
Have a great school-year!