Memorizing has, for years, been very fascinating to me. I think that's partly due to the fact that it was such a struggle for me in the beginning, whereas now it seems like a very natural thing to do. It's something most of us haven't really been taught, but expected to figure out by ourselves - if we don't learn the piece by heart, we're usually just told to practice more.
I find it interesting how such a crucial part is just left for fate do decide.
When starting to play piano, memorizing is usually not such a big problem. The songs or pieces are short enough to learn just by playing them through a couple of times per day. But once we enter even slightly more advanced repertoire, that way of learning is suddenly of no use, and we tend to find ourselves fiddling around on the keys with an increased heart rate, and with the words "Oh, shit" repeating in our heads.
I was no different. I easily came through the harmonically simple and motorically repetitive pieces without any difficulty, and putting my head on auto pilot was my idea of a perfect concert - I'll get up on stage, bow, sit down and start to play. Things will then turn black, and I will come out of the darkness a few minutes later once the last note is played. "How did it go? Oh, just amazing, I didn't even have to think!"
Then it came.. I went up on stage, bowed, sat down and things turned black. "But what was that note again", and it all changed. I came out of the darkness, but I wasn't even half way through the piece. Desperately I tried to find the right notes to play, my heart rate increased and "Oh, shit!" was on repeat in my mind.
Luckily, it was on a sort of bad piano, and the audience was a bunch of classmates who just wanted to go home. They were probably coughing and yawning a lot, so it was surely because of that, and nothing I had done wrong. All I had to do was to practice some more and just focus a bit more next time.
The next concert came - I went up, bowed, sat down.. all just routine by now. The only difference was that this time, I'd be Focused!
"Focus, focus focus focus... fo.. f.... oh, shit!"
These slips went on. I could play it perfectly at home, and almost as good in the lesson room, but once on stage, it all just fell apart.
I went on to practice 8-10 hours per day, but still with the same result in concert.
I got some tricks from my teacher, like "Think of the harmonies" or "Practice more slowly, and with separate hands!", but that rarely made any actual difference.
The real difference came when I started studying at first actual music school, when I was about 18, where other students also had to practice. The rooms quite quickly became full, and there was a lot of time spent just waiting. As the pieces had to be learned anyway, I started by just reading the score, and tried to imagine what it looked like. I read a line, or a phrase, closed my eyes, and tried to play it in my head. Not only was I unable to remember the notes, I barely remembered what it sounded like! It was such an epiphany that I knew it would change my musical life. I finally started thinking about practicing.
How would I ever be able to play anything if I had no idea what I was actually playing?
From then on I started practicing away from the instruments at least an hour every day. I tried to not allow myself to practice at a piano before I had a very clear idea on how it was going to sound once I started playing.
It became clear to me that the motoric memory (or muscle memory, as some calls it), the thing I had used for so long, was actually very unsafe and unstable.
I found out that the other senses have an impact on our memory too - the aural and visual, and the intellectual part - and how they, unlike the motoric memory, really needed to be worked on properly and wouldn't just come by themselves.
I started analyzing my performances, and what I was thinking during my playing. As it turned out, I was thinking almost exactly the same things when I was performing as when I was practicing, and once the muscle memory turned off, I had no safety-net since I didn't make one when I was practicing.
Practice, in some aspects, shouldn't really be that different from performing.
Once I started practicing everything I had to know on the stage, all the memory slips disappeared almost over night. I finally realized not only to focus, but on what I should focus.
If I noticed that my mind was drifting on a certain spot, I practiced to know what to think on that spot (the notes, the character, to sing the melody...) to train myself to think about the same thing during the performance.
Or if I noticed that I worried about remembering the notes in a certain phrase, I practiced every voice carefully in my mind, both the notes and the music, before I played them, and followed each of them very attentively when I played. Then I would have created a safety net, in case I would need it in public.
I practiced very attentively like this for some time, to make sure I would cover the full piece - I remember once when I was performing the Symphonic etudes by Schumann and, with full focus, was able to follow everything, down to pretty much every single note - but then got into the next problem.
You. Can't. Make. Music. If. It. Never. Ever. Flows. ... Which is what happens if you think on a note to note basis.
It was an obvious improvement from being neither detailed nor attentive, but it was a few steps too far.
I tried to alternate between being in constant control, with often playing through to put everything in a certain perspective.
Now I feel that I can be both while performing. If I feel I am under control, I can let go and let the music flow by itself, and can just as easily go back to regain my focus if it's starting to fall apart.