One thing that really helped me decrease the amount of mistakes I have in my performance was increasing my sight reading ability. I did this by accident ^^
When I first found sheet music online, I kind of went piano crazy, because I never knew that there were sheets of so many songs that I have heard and enjoyed! I found tons of anime/video game themes that I have loved xD I printed out over 50 songs and would play through them all several times, not because I wanted to improve, but because I wanted to play the songs. For the first time in a long time, I was really enjoying the music I played ^^
After playing through so many different pieces that I enjoyed, I became much more expressive in my playing, and an improvement in my sight reading came naturally. Now whenever I play pieces, I don't focus on the notes, but on the song itself. On enjoying the piece, for example, this one I liked a lot:
http://www.youtube.com/watch?v=Tjz7ILFnNMY&feature=email
(Played all the right notes, but sounds weird in one part. I think sheets were wrong xD. But anyways, playing musically is much more important than playing all the right notes, and when you play musically and passionately, the notes will come naturally ^^)
Of course, when I practice, I'll focus on learning the notes, but when I play, I just try to create music that I enjoy ^^
Also, I found that closing your eyes and playing makes it so much easier to hear the song rather than just notes. (This may be awkward at first, it definitely was for me, but believe, if you know a piece really well, you'll be able to play it eyes closed, and you'll be able to hear it at a whole new level ^^)
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So, I guess my suggestion is, find some songs you really like and play them because you love the music ^^