Motivation is not something the teacher does for the student. It is something the student does to himself/ herself. The teacher's job is to present the possibilities.
Give students as many and as varied decisions as they can handle.
Besides studying the classical pieces, give students the tools for pick-up other music. For example, jazz, hymns, movie themes, Broadway tunes, and holiday music. These tunes are great 'concert' material for family and friends.
Keep in mind the student's goals for piano study. If the student feels that his/ her study is moving toward these goals, motivation will stay high. It is open to have piano competition and/or examination as other ways of goals.
Demonstrate the student's progress. Make sure he/ she sees the progress he/ she made. Exercises are another good way to demonstrate progress.
Demonstrate teacher's confidence in students. Let the student know that the teacher approves of and encourages his/ her working ahead. Teacher should reward initiative.
