First, let it be known that this idea is not original with me! I found it online and can't remember where, so if you posted this idea somewhere, let me know and I'll give you credit! (UPDATE: It was Marcia Vahl over at Piano Perspectives! Thanks for this great idea!)
One of the most successful tricks I used this year with my elementary piano students was mystery cards. I made 12 cards, each with a different behavior that I want to see all of my students display. At the beginning of the year, I gave out a list so the students would know what I expected. At each subsequent lesson, the student drew a card from the deck, but couldn't look at it. I did look at it, and then watched throughout the lesson for them to demonstrate that behavior (or not!). If they did, they earned an extra music buck.
I often forgot, but the students never did. They'd stop me in mid-sentence and say "Mystery Card!" When a student started doodling on the keys while I was trying to explain something, I'd just say, "I hope you don't have that doodling card..."
I made these up on Word and printed them on card stock and let one of my students cut them out. If you want to be all neat about it :-), you could print them on business card blanks.
Here are the "signs of a good student" that I used on my cards. Maybe you can think of more!
1. I remembered to bring my notebook and my music.
2. I kept my eyes on my music and not my fingers.
3. I used good piano posture for the entire lesson.
4. I played all dynamics correctly without being reminded.
5. I played slowly and carefully (when appropriate, of course).
6. I played all note values correctly on assigned pieces.
7. I passed on all my assigned work this week. (comes with the understanding that assignment may have been to play hands separately, or to play only a portion of the piece)
8. I didn't make any excuses.
9. I had a positive attitude.
10. I used a great hand position for the entire lesson.
11. I adjusted the bench before I played.
12. I listened well without doodling on the piano.