Hey Bill, here's an idea that works for me. To begin with I think you should write the students homework in a notebook so they can clearly see what you were doing with them in the lesson at home.
It will fresh their mind and organize your lessons better since you'd be able to follow the students progress by looking back in the notebook.
I don't know how many students you have but if you have more then three it gets harder to remember exactly which one played what.
In that notebook draw a table with blocks. Each black worths 10 minutes, 2 block per day.
Once the student has practiced for 10 minutes he's aloud to paint the block in black.
If the student didn't practice on a certain day tell him he has to mention why he didn't practice. Tell him he can write everything and you won't be angry. Even if he just didn't feel like practicing.
The impact of the blocks is amazing. I'm performing this precedure with my students and they love filling the blocks. It really encourages them to practice longer.
If they don't practice often they will slowly be ashamed from their excuses without you preaching them. It's important they will be motivated by their own integrity and not by a being rebuked.
Students practice more when they love their playing material... but that's another subject.
You probably do well but just thinking about it is important.
If you notice that your student is messy by filling the blocks, you can help him to learn how to arrange his schedual so that he will have enough time to practice. Students love limits. They really need it from us teachers.