Music lessons aren't cheap. Students are paying for the undivided attention of someone with years and years of musical experience. The best way to ensure they get the maximum value out of this interaction, I think, is having a clear vision for the student's future.
The student needs a goal that glitters to them. They should have an image of themself playing their dream music in their dream situation that makes them tingle with excitement. Where are they playing? With whom? What kind of music? What does it sound like? They should have players, even specific recordings, that make them say "I want to be able to play like that!" The teacher should give the student (and the student should seek out for themself) recordings of different performers and composers to listen to, to find out what they like listening to and who they want to imitate. The more clearly a student can imagine a musical future for themself that authentically excites them, the more motivated they will be to practice and to continue taking lessons, and the happier they will be.
Once the student has created this vision, the teacher's job is to figure out what the student needs to do to get most efficiently from where they are to where they want to be. The teacher knows more than the student, so they can suggest things for the student to practice. But the more specifically they understand what the student wants to become, the more valuable their suggestions will be. Together, the teacher and student should come up with a week-by-week action plan. The more the student has a role in assigning their own practice goals, and the more they can see how what they're practicing will lead directly to where they want to be, the more likely they'll be to practice.
As time goes on, the student's unique vision for their playing will become clearer and more particular. This is a good sign, because it means they are continuing to change and discover new music and new musical horizons. Because of this, the conversation about the student's goals should be ongoing.
Having a clearly articulated goal should help the student draw the most out of the teacher's well of knowledge. That way, the lessons will be worth every dollar. Personally, it makes me feel good knowing I'm able to offer a service to the world that's of the maximum possible value.