My most significant breakthrough moments were unpredictable and highly personal, in the sense of being very specific to a particular time in my overall learning process. Here they are, in roughly chronological order (as best I remember).
The last item is the one most important for me, and which I unreservedly recommend. You could read that and skip the rest. However, it happened after all the other things listed, so I must allow that at least for me, it depended on them.
Moving from an upright to a grand piano
After practicing for some years on an upright piano, I took a lesson with a teacher whose first question after I played was about the instrument I practiced on. He told me I'd maxed out the capabilities of my piano and that I needed a grand.
He was absolutely right. The difference in sensitivity to touch, the greater range of volume and tone color, had a near immediate effect on my playing, because I could better hear and imagine nuances to the music I was studying.
Taking two-years of college-level music theory -- twice
The first time through the curriculum, I learned quite a bit, but it was the second time around that the theory and ear-training really came together. The effect on reading, learning, and playing music was immense. I was able to better recognize, both at sight and by ear, musical phrases and how they related to each other. It also increased my ability to conceptualize the music in ways that made memorization much easier.
Artur Rubenstein's recording of Schubert's Impromptu in Ab Major (Op. 142 No. 2)
I had been playing this piece for some time -- fairly well, I thought -- and performed it for a friend. When I got to the middle section, the friend started yelling at me to "Go!" I was "going", so I really didn't understand him until I stumbled onto Rubinstein's recording. At the B section, Rubenstein speeds up suddenly and dramatically. I was so startled I started laughing. That was my introduction into how much room for interpretation and expressive breadth music could accommodate.
I spent quite a few years looking for ways to promote relaxation in my playing: a teacher who specialized in it, yoga, massage (several different types), physical therapy, meditation, perhaps others now forgotten. All of that came to a head when I participated in a week-long (or two weeks) Dalcroze workshop. It was in that workshop that I realized I was restricting my movements, and it helped me greatly expand my understanding of the relationship between how one moves (at the piano) and what one expresses. It gave me a much greater appreciation for the breadth of physical movement that is possible.
Playing easy music -- a lot of it
When playing Debussy, Schumann, Chopin, Rachmaninoff, Bach, ... whomever, there are a huge number of things to keep track of: notes, rhythm, articulation, dynamics, voicing, fingering, phrasing, etc., etc., etc.
The absolute best way I've found to improve in any area of my playing -- reading in particular -- is to play the easiest music I can find. I've read and practiced my way through several "Book One"s several times each, and continue to do so.
The benefits are too numerous to list, but it boils down to the fact that by playing such simple material, I can focus in a detailed way on all of the musical aspects simultaneously. When I read, I can read all of the instructions at once while simultaneously building an interpretation. When I practice a particular moment, it's obvious what the essential elements requiring practice are, so I can focus on them without struggling with additional complications.
I use a three-step approach:
- Read through to get a feel for the piece/exercise.
- Step 2: Practice until I can play with fluidity and expression.
- Step 3: Make it really sound like music.
This is harder than one might think. The musical material is so limited that making music out of it is exceptionally difficult. In that respect, Chopin, for example, plays itself as long as you just follow the instructions. But try to make a two-finger exercise using only middle C and treble G sound like Chopin.... To that end, I try to envision playing for an audience of pre-schooler's, and I have to get them engaged in whatever music I'm playing.