Choose appropriate repertoire that is fun for the student.
Let's face it, any student, even the most motivated, will hate lessons if they hate their pieces. They won't practice, they will dread coming to lessons, and they won't progress. The trick is to pick pieces that are fun and enjoyable for them, challenging enough to boost their confidence and improve their skills, yet not so challenging that they get frustrated.
The best way to do this is to become familiar with a lot of repertoire yourself! Listen to lots of repertoire. Pick up a book and play through it, noting which pieces are fun, what types of techniques and musical concepts they teach, and what level of student they would be appropriate for. I often like to give a student a CD to listen to which includes a few possible pieces to learn. This way, you have some say over the pieces and can pick some that are at a good difficulty level for them, but the student gets the final say.
(Choosing repertoire is so important, and is partly why I started the Weekend Repertoire feature here on The Teaching Studio!)
How do you go about choosing good repertoire for your students?