I grabbed this idea from The Creative Apple. Have your students come up with ideas of things that they would enjoy earning, with your approval. Write down the agreed upon privileges and place them in a jar. When you feel a student has done something EXTRAORDINARY, you may allow them, (not always) to draw from the jar. This is a great motivator that will keep student's striving because they'll never know when they may get to choose a special privilege. On the occasions they do something awesome, but aren't allowed to choose from the jar, make sure to let them know how wonderful they are and to keep up the great effort, because next time, may be the time they get to choose.
When initially presenting this to the class, make sure to let them know that their awesomeness will always be appreciated, even if it doesn't result in a drawing. Let them know there wouldn't be enough time in a school day to reward for all the great things they do on a regular basis, but this is your attempt to give back to them once in awhile for all they do. ;)
The jar may look something like this...
