My approach with motivational reasons for poor handwriting has been natural consequences. If you or your teachers can't read your handwriting, resulting in you not receiving full credit for work you did (or, as some of mine have done, you propagating errors because you couldn't read your own handwriting), then you will just have to make choices about how much you value the accurate communication of you and your work to others. The purpose of written work is to communicate and record your thinking and work to other people. If it is not effective at communicating, then it is not fulfilling its purpose, and it makes sense not to credit you with having done the work or the thinking.
