Intrinsic motivation

Any teacher will tell you- one of the most difficult things to do is to motivate the unmotivated. What do you do with a student who just doesn't seem interested in doing their work? Or, even worse, who is capable of so much more, but just doesn't put in the effort? How do you motivate them to achieve to their potential?
Elementary teachers have a variety of tricks up their sleeves, but occasionally, you encounter that student where none of your tricks seem to work. You end up doing backflips and cartwheels, singing and dancing and doing just about anything you can think of. Sometimes you get lucky. And sometimes you just don't.
I've encountered a similar problem recently....not with a student, but with a student teacher. For the third year running, I was asked to host a student teacher in my classroom. It's probably no surprise that I take this job very seriously, after all, this person will be launched into the education world with the practical skills they gain under my guidance. And I do mean practical. You can read all the theory books in the world, but you learn more from your first few days of teaching than any amount of schooling. You have to do it, to learn it.
Despite my gentle encouraging to be an active participant in the learning process, my most recent charge spent much of her time sitting. Now, this is not something she learned from me. I'm lucky if I manage to sit down for five minutes during snack. Upon seeing this, I chatted, kindly, with her about conferencing with the students during reading and writing workshop. The next day, I was pleased to see her sitting next to a student and chatting. For twenty minutes. My pleasure was short lived, especially when the very next day she returned to sitting at her desk. The last straw was when she taught her second ever spelling lesson sitting behind my desk. I was speechless. I was shocked. I was in a state of disbelief. I didn't know what to do.
Recognizing this as an opportunity for professional growth (because I had to refrain from telling her what I really thought and instead find a way to make her understand that this was unacceptable and it better never happen again) I sought the advice of a veteran colleague. We talked, she gave me advice and I listened.
After a lengthy conversation with my student teacher that involved me pulling out every sneaky conversational trick in the book, she came to understand that sitting and teaching just isn't a good idea. I waited-anxiously-to see her next lesson. Relief. She was more organized, better planned and on her feet. Thinking I had solved the problem I mentally patted myself on the back and life in my third grade classroom continued.
For the most part, my student teacher has improved. As she's taken on more responsibility (which I admit I have been passing off very reluctantly. Control freak? Absolutely. Especially when it comes to my students!) she has gained more confidence and I see her style developing. But my goodness it scares me when I ask her if she needs the book for the weekend to plan her lessons and she tells me no.