Please share your memorization techniques for piano music. I'm good enough at sight reading to have avoided learning how to memorize effectively, which is now holding me back on tougher pieces. For such music I need to transition from constantly looking at the music to looking at my hands (at least sometimes), when I increase the tempo. I practice 2-3 hours a day, usually in two sessions, usually 3-5 times per week. I've tried visualizing the notes but that's so abstracted from the actual music and my tempo already fast enough, that I can't visually recall and process in time - especially for pieces with many concurrent voices. I also have more difficulty memorizing contemporary pieces which I don't understand theoretically. For example, Bach preludes and fugues are much easier to memorize than Shostakovich preludes and fugues. In time actual mistakes made tend to erase my visual recall. I also try "air piano" (playing on a flat surface) but without the sheet music in front of me I don't improve.
Edit (in response to @Tim & @David):
I try to minimize mistakes during practice. If I take a piece I can play perfectly with sheet music and try to play it from memory, the mistakes accumulate until I can no longer play it perfectly even with sheet music. In fact the approaches outlined in Answer: What exactly do pianists/musicians memorize? all seem to lead to this outcome.
Also, I'm as close as can be without actually being tone deaf. I'm pretty good at telling when two notes are slightly out of tune, but in any other situation I can't even tell which note is higher, even with octaves. It goes without saying that I can't carry a tune.
That said, I agree that I must be internalizing something auditory and melodic during practice. I sometimes hum along, though I've been told (though I can't tell) what I hum is completely discordant with the actual music. That said, I learn such pieces much faster.
What to hum for more contemporary music? For example, I have Prelude No.2, Op.87 (Shostakovich) up to half speed. That's fast enough to think of each half-measure as a single chord. Except subsequent chords change just one or two notes. I don't think the approach outline in Answer: How do you remember your music and how do I improve in this regard? applies; there is no standard pattern or melodic shape.
I'm currently playing that piece just once each hour with sheet music (only on weekends, unfortunately) in the hopes that will increase acquisition. That said, I don't think repetition and muscle memory are completely dependable. I play smoothly until I stumble over an unexpected section. Rather than my muscle memory having failed; I suspect my conscious brain broke the concentration of my muscle memory. In either case I suspect memorization must also include some form of conscious future expectation be it melodic or visual.
So I've memorized that piece and can play it perfectly without sheet music. While there might be a shape component to the memorization (i.e. the shape of the arpeggios) in addition to a melodic/auditory component, I find no visual component. I don't look at my hands and expect a certain finger to play any concrete upcoming note. If I get lost, I "consult" the shape of the arpeggio in my mind to then determine which note to play next. And since my eyes aren't any help - except when needed to calibrate jumps - I can pretty much play blind.
This has helped me play the piece using sheet music. In fact with sheet music I can play at full speed (excluding those four difficult measures). Unfortunately and despite constant and extensive daily practice my memorization, as usual, seems to be deteriorating. Despite my best efforts and slower tempo, without sheet music I make more and more mistakes. I actually play better if I look at music of a different piece, or at least away from my hands. Worst of all, this always happens. The most common mistake is either to play the wrong measure or to forget what follows. The second is to make disparate connections between distant measures. If both measures contain the same sequence I'll unconsciously splice them together, ignoring everything in-between. How do I stop this?
My current theory is that I use the sheet music as a "focus point" in aiding my concentration, which without I simply zone out. Contrary to one answer, despite focusing on sheet music I have full control over its interpretation (dynamics, speed, etc), though I am unable to improvise or transpose the music.