One of my students always had special songs to prepare for his grandmother's birthday or something. Both the grandmother and mother were very involved, and routinely dropped hints about their favorite songs.

Very young children (ages 3-8 or so) respond to stickers or checkmarks for each day. I never understood it, but it really works.

I think I read somewhere that truly gifted protegies cannot be kept away from the instrument...but there are few Beethovens around.

In looking back at the original post, motivation doesn't seem to be the problem, but rather, the problem is practicing regularly. This may be more of an organizational problem than a motivational one. Some people are naturally organized (my daughter), and some aren't (me). There are aids for folks like me, that once they are in place, life is easier. schedules, lists, calendars, tangible rewards...structure.

Teachers in dyslexic schools know a lot more about these tricks. Organization and routine are the key ideas.