Let me copy paste something from my private blog (this was in January 2016), where I describe my personal memorization technique (I was influenced by Chang's book at this period; though I disagree with a lot of what he says about technique, I found his chapter on memorization very useful):
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There are 5 types of memory a pianist uses: Hand memory (muscle memory), Music memory (the music tells you what to play next lol it’s really efficient), Keyboard memory (the ACTIVE remembering of what keys you’re about to hit - a litmus test if you can see a piano in your head and see the notes being played in your head, also known as mental play), Theory memory (memorizing structure/shape/harmony of the music), and finally Photographic memory (of the sheet music). The last one is a spawn of the 3 before it.
All of these memories are very closely related, though to go between them, you must do according to the arrow on the chart:
Not the best drawn picture, I know. But what I realized was I was using musical and theory memory the most, and I forgot one of the most important parts of the picture - keyboard memory! I was relying on a BUNCH of repetitions to create my keyboard memory, but I realized today, during class, that I could create the theory memory in my head, away from the piano. Combined with the other memories, it makes memorizing EVEN faster, and not only that, it makes playing from memory a lot smoother.
So what I’m doing right now is I’ll look at a bit of music, memorize it theory wise, then close my eyes and create the keyboard memory in my head, and THEN I’ll play it for the first time. This works quite well, I must say.
That reminds me; keyboard memory is crucial to have at all times, so long as it’s not distracting you from your interpretation. In other words, you have to spend as little mental focus on it as possible, but it must still be there (and be guided by your music memory). Hand memory is crucial, but you must pay no conscious attention to it.
Also, I realized a few skills I need to develop: mental play of something on the keyboard after just listening to it, and, the key to sight reading - looking at “notes” on a paper, and then immediately visualizing “keys.” So long as you have perfect mental play - you’ll never mess up! There are two kinds of mental play - visible and invisible. Visible mental play refers to playing it in your head whilst being able to see the keys. Invisible means playing it in your head BASED ON FEEL, but you know exactly which keys you’re pressing. IT’s kind of weird.
tl;dr, use mental play (keyboard memory) in addition to everything else, don’t neglect it!
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So here's my personal memorization technique as of right now:
1. Listen to the piece on repeat until I can recall the entire thing note for note in my head (Warning: you might go insane; I did when I was doing this with the Rite of Spring)
2. Obtain the score, and analyze it to literal death, make sure you know exactly what is going on in terms of the architecture, harmonic structures, melodic structures, appreciate the counterpoint if there is any, revel and bask in the piece, and just /study it to death/
3. (This step can be done concurrently with step 4, but I prefer to do it beforehand) Create "keyboard memory;" visualize yourself playing the piece in your head. I find it much easier to start in little chunks, hands separate, but see the shapes on the piano, and see the keys being depressed in your head. To make this even more accurate, feel your hands playing the bit in your head. (Though you must always check on the piano to make sure the motion is accurate!)
4. Actually play the darn thing; here I do a lot of bar by bar, 2-bar by 2-bar, section by section practice, where I'll play the same bar like 20-40 times before moving onto the next one. Literally, as much repetition as possible in as small of chunks as possible. As the days go on, I make the chunks to be repeated longer. /No mindless repetition; must be played with purpose/. I also try to see the notes in my head a split second before I play it. I might try blindfold practice here and there. (Visualizing the piano in my head helps me hit jumps with my eyes covered for some reason)
Note: Usually when I am at this section, I already have the entire piece memorized theory-wise, and usually I can see the whole thing in my head already, so it's just a matter of fleshing out the motions. I separate steps 3 and 4 because to me, learning the notes and learning the physical motions was too much at once, so I separated them! (For more difficult music, that is)
5. Here and there, I'll pull out some staff paper and attempt to recreate the score from memory. You'd be impressed how good you are at this.
That's just how I go about it though. Best of luck!
Topic: Help! Forgetting my piece when performing!