Rocks in the Basket is widely used at Living Wisdom School for encouraging positive attitudes and intrinsically-motivated good behavior. It is a very versatile, flexible, and easily implemented technique that is used for pre-K through 8th grade children.
Each classroom has a basket and about 100 or so rocks. Whenever something positive is noticed a teacher or child will put a rock in the basket. When the basket is full, the class will have a celebration.
While a simple technique, it's implementation has important subtleties and its impact over time on children is profound.