Don't slack off. Sometimes it may become tedious, but keep practicing.
Don't be nervous at recitals. Just play your piece. Don't worry about how you look. Pay attention to the thing that really counts -- how the music sounds!
Don't buy or let your parents buy a piano when you're not sure you want to start playing it at all!
Never play the same melody the same way. If the composer puts identical measures in the piece, make it interesting by using dynamics or ritardandos.
Don't settle to just play the notes on the page. Think about what the melody is trying to convey and play the song as though it was your own - from your heart.
Don't set yourself impossible targets. Don't just say 'Right, I'm going to learn how to play Turkish March in one week'. It's almost impossible.