I have found a great way to get my kids to practice piano. We've told them that for every song they pass they get 10 minutes of free-play piano time at their next practice.
I'm realizing that this motivates some more than others.
The oldest one (aka "The Rule Follower"), who needs no motivation, has a hard time figuring out what to do for free time. He sits there, frustrated, unsure what to do without the book. (We do let him look at the book, he's just created a rule for himself that free time should not involve the book.)
The youngest one, on the other hand, loves his free time. This is the same boy who, a month ago, complained any time he had to touch a piano. Today, during twenty blissful minutes of free time (he passed two songs yesterday!) the youngest announced, "I know what I want to be when I grow up! I want to be a piano player and someone who writes the songs."
Moments like that make my heart happy.