TLDR VERSION:
Teachers need to determine what drives a student to work. Once we have worked out their motivations sometimes we need to encourage changes to these if the student hopes to ever make effective progress.
LONG VERSION:
How difficult is it to start to do something every day that is out of your normal routine? Of the many students I have taught over the years I find that improving students work habits are one of the more difficult achievements.
Students that work consistently make the most grounds. I find some just love the act of learning new things and mastering old skills, they are strongly knowledge/achievement driven. There are some who love music so much so that the passionate/spiritual connection they have with the music drives them to work hard at it. A few are motivated to do well because of the money/material they can make from it. Then there are some who are forced into a strict regime often set by the parents but of course not excluding the examination student or music major who must abide by a syllabus.
I find that not all my students necessarily fall into ONE group or the other, there often is a combination of motivations encouraging them to work consistently, I find these ones make the best progress. As a teacher I can sense what motivations are more important to the student and often I need to make decisions as to whether or not they need to improve upon that.
For example: an excited student telling me that they need to learn this piece (which is too difficult for them) simply because they "love it". As a teacher in my mind I need to work out whether or not they are really serious when they say they "love" the piece, just loving it is not enough, are they really willing to work hard or do they just want to hack their way through it and have a brief messy affair with the piece? Often I will let the student learn the piece but it never becomes a main focus of the lesson, we will do bits and pieces only. If they are really serious they will learn easier pieces which will build them to be able to play the piece they love with less problems.
I have to admit however that some students are with such determination!! They insist in playing a piece too difficult for them so I take them through the process. A few times we have achieved wonderful results but most of the times they are left with difficulty upon difficulty and they cannot cope with the amount of practice/progress required or cannot deal with the slow learning curve. I find at this instance when they realize the work they love is too difficult/inefficient for them to learn, if they need to have other motivation to learn, I am yet to find a student then being so stubborn to NOT take my advice to learn something easier and build towards their goal. Most of the student "get it" and thus we have changed their source of motivation. As the teacher I allowed them to "test the waters" of the piece they love I showed them the amount of work required to do it service (or to play it at all!), once they realized they have been too ambitious they then make changes in what drives their motivation. They needed to try first at least I find most students need that, just to try it, fail, then work out how to get there with the teachers assistance. No longer are they simply playing to learn a piece they love, but they are excited as they work towards being able to finally play the pieces they love, there lies a big difference.
It is not surprising that I find that many young students like to get rewards, stickers or lollies! It is not bad to bribe your students with these at least they get a sense of achievement and a sticker to a child is worth a lot don't you remember?? But as they grow up this sort of thing gets old. I find after a while this system of giving rewards becomes less effective at motivating a student to do work, its "luster and shine" wears down.
The student needs to find a stronger motivation to do well with their piano (or anything else for that matter). If I as a teacher cannot encourage the student to make that connection with their music then they will never progress. I find that no matter what some students simply never will change their mind.