Motivating students to practice - my ramblings

Motivating students to practice is no easy task.  As part of my class, students are required to perform playing tests for me every 2 week, and that is supposed to be motivation for them to practice, yet still, students don't always do the work and I am left with some students who lack the skills needed to be successful in my class.  I lecture them about practicing.  I try to teach them how to practice effectively.  I show them videos of performances to help with desire to be successful and practice.  Still, it can be so frustrating when I see that the progress of my orchestra is slowed because of kids who don't practice.  Recently, I asked my students how often and how long they practice as a bellwork assignment.  I was astonished at the amount of practice that was NOT happening!  No wonder my large class of beginners feels like such a struggle right now!  What to do?  Require practice cards?  I don't know.  I want my students to practice because THEY want to practice.... I don't like to force them.

I am still trying to decide how to motivate my class.  I am currently holding a practice competition where my students are recording their practice times on a giant paper in my classroom.  The class with the highest average gets one playing test cancelled. 

I think I might bring a potted plant to class.  I will have students stand-up if they practiced the night before and I will count how many students practiced and I will mark them on my seating chart so they see me noticing and marking their efforts.  Then, I will water my plant a teaspoon of water per student who practiced and we'll see how my plant does.  My plant will die if not enough students practice.  I'm thinking this could lead to a great object lesson about nurturing your talent so that it can grow.