Does your child enjoy an activity that they have the privilege of doing every day? ex. Video game, watching TV, iPad time, Computer activity, etc.
Simply inform them that from now on, in order to earn the privilege of that activity, they must first practice piano. And guess what? It can be a positive experience! "If you practice piano, then you can enjoy your favorite activity!" It can change their weekly pattern of arguing about practicing and disliking piano to a new pattern of daily practice and genuine excitement about checking off their song in class. It's been, by far, the most practical and effective piano practice incentive we've used!