I think Joe's tactic is very interesting. I have a related, but different problem, with students and essay writing. They think that they are writing for themselves, and it comes as a shock to them when I point out that the whole essay format is designed to present a tool for other people to use. They have been taught self-expression for so long (as a way to get them writing in high school) that the idea that someone else would read it, or need what they have written is quite shocking. It is a real mental "paradigm shift" -- read this as if you were someone else. It seems that in the performing case people get so much pleasure out of being immersed in playing the music that they have no idea what an audience is hearing. It is interesting because obviously you want to have both -- pleasure in participating, and also giving pleasure; but the shifts back and forward over that divide obviously require some work for some people. I never thought about it in music before.
yours, Peter T.