Here I am talking about advice from a teacher, or even a fellow student, who gave you a piece of advice which saw a marked improvement in ability or attitude.

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The one piece of advice which made a massive difference to me was quite early on in my studies. I was playing the Minuet in G - by Bach. I thought I played it ok - the notes were all there-and it was in time and I thought it was perfect. I expected praise. Instead my teacher sat down and played it so beautifully, it was like I was hearing it for the first time.
She told me that before I learn a piece of music, I should sing through it - I should be able to sing every note - where there are multiple parts, I should sing them seperately. In this way we begin to see the melodic structure and the harmonic structure also. Then when we play the piece we should sing it with our fingers, as well as with our hearts. I understood thenm why Glenn Gould sang as he played!
When I could sing the Bach Minuet, and remember it perfectly - my playing of the piece came alive - and I was more sensitive to bringing out the melodic motifs and the sublime little modulation to E Minor, became a thing of awesome beauty to me.
As my studies progressed I found I could remember far more using this method - and even complex pieces which even a virtuoso singer wouldn't attempt -or be able to pitch - can be sung mentally -if not literally - it always helps me.
Anyhow, what is your most treasured piece of advice?