The Play It Forward program encourages our high school age players to volunteer hours in helping our younger teams. They help run practices, throw BP and can even help during games. The original idea of this program was to have the older players "give back." An awesome side benefit is how much it has helped the older players to love the game and in some cases discover leadership qualities.
We log hours and have a competition to provide an incentive to participate, but we have found that the service itself is the best incentive.