I don't have any words of wisdom to offer, as I only started memorizing pieces seriously a decade ago (because I started performing from memory), and currently only have about three hours of music in my memory, of which 2/3 are secure enough that I can perform them anytime, anywhere, whereas the rest need a quick refresher if I intend to program them. (Prior to that, any piece I could play from memory got in there by accident......)
My methodology is always the same when contemplating learning a piece to play from memory: do I want to keep the piece in my rep forever and continue performing it regularly (i.e. do I like it enough)? If there is any doubt, I don't memorize - I just learn it to play for my own satisfaction (or even just sight-read it, then play it a few times from the score, then discard
). Maybe later on, if I really, really, really want to perform it, I'll memorize it......but very few pieces go that way. My first instinct is usually right
If I decide at the outset that I want to memorize, I'll start learning it that way. An initial sight-read through to determine which bits need work, or fingerings to be sorted out, and which bits are especially difficult to keep in the memory (the two don't often coincide). Then I start memorizing while learning, one short section (a phrase or two, maybe a whole page if it isn't too complicated) at a time, starting at the beginning (always a good place to start, as Julie Andrews told us). From then on, I always start and finish every practice session by playing what I've learnt from memory: if my memory fails, I go back and make sure I get it right before I close the fallboard (though my piano hasn't got a fallboard). It takes as long as it takes, and I use every trick in the book (plus tricks outside the book) to help/jolt/imprint my memory: patterns (in the score as well as at the keyboard), hand & finger positions on keyboard, 'feel' of keys beneath my fingers, 'shape' of my hands in any particular section, harmonic progressions, accidentals, modulations, rhythmic quirks, signposts of any sort that help my memory. And of course, once memorized, I make sure that I can start playing from any of several points in the piece: essential if I have a memory lapse. Therefore, I might start a practice session by starting from one random section that comes into mind, rather from the beginning, and keep playing through to the end, or stop and start again from another section. Or even stop and play a completely different piece, then switch back to the one I'm learning with hardly a pause. Sometimes, when I'm in the mood, I'll switch back and forth between two or more pieces, playing a section of one piece, then jumping straight into a section of another piece, just to check my memory
But the bottom line is, most of my playing memory is muscle memory. (And it works well, and especially with fast pieces with lots of notes, which are the ones I prefer - 90% of my performing rep are fast or faster.....)
As one well-known pianist puts it: "If you play a piece enough times, you remember it."