If you mean mistake free as in no wrong notes, I think the answer's obvious if you know how to make exercises for yourself. Now if you really mean mistake free, you just have to really listen when you practice. Record yourself if possible. But be warned, there is rarely a performance that the performer could listen to afterwards without having any critique.
Geez! By some of the posts on this forum they all do! I seem to be the only person here who actually spends months learning a piece. These guys just wake up one day knowing them all!
How often do I give recitals that are "spanking perfect" ? Never, and I'm not so sure I ever will. If you were to ask a great pianist, I'm sure many would feel the same way.
Somehow, I also got that feeling..