After I've given a concert, the question my audience members ask most often is "How to do memorize all of that?" Luckily, I've always had a good memory. But I don't rest on my laurels; I work at it every day. More importantly, I want to help my students with memory issues, so I've made an in-depth study of the subject of memorization as it applies to piano.
Some musicians, or people learning to play an instrument, memorize easily, some do so with a good deal of effort, and some find they can't memorize at all. The fear of memory slips, and the embarrassment of having them when playing for others, takes a lot of the pleasure out of playing in public. People who memorize easily often can't explain how they do it. For those who memorize with effort, it takes a good deal of time, and still causes anxiety and fear of memory slips.
In playing an instrument, there is not just one kind of memory, there are several: these are: "muscle" or kinesthetic memory, auditory memory, visual memory, and what I might call "intellectual" memory.
Muscle memory is what happens when you perform a physical action, or series of physical actions, repetitively. One almost cannot help developing muscle memory, but it is stronger in some people than in others. It certainly is an important tool. For my students who are beginners, I help them develop muscle memory by urging them play the music with the same "fingerings" (I prefer the concept of hand positions) consistently, so the body is not confused by doing something several different ways. Muscle memory requires a certain amount of repetition of course; this is why musicians assume they must practice hours and hours with hundred or thousands of repetitions. However, if you practice smarter, you may not need so many repetitions as you think. Instead of playing a passage over and over, play it a few times but focusing intently on how it feels to your hand, arm and torso to play it. Muscle memory will grow faster with that kind of attention. Muscle memory alone, however, is not enough. It may be somewhat quick to acquire, but it is also quick to lose. Lay off practicing the piece for weeks or months and you may find the muscle memory weak, if not gone altogether.
Auditory, or "ear" memory, could be described as the internal image of how the music sounds. You may think that you have a clear auditory memory of your pieces just from listening to them. Again, some people will pick up auditory memory quickly and others struggle. It is directly related to how strong, how well-developed, your ear is in general. A good test to see how strong your auditory image is, is to transpose the piece. After all, if you really know how the music sounds, it shouldn't matter what key you play it in, right? (A simple example of this is singing; you may not know in what key you start singing "Happy Birthday," but you don't forget how to sing the tune.) Can't transpose the whole piece? Try just the melody to start. It may be harder than you thought. If you can't do it easily, you don't yet have a clear auditory image of the music. Keep transposing, however difficult, and it will strengthen the ear memory. When this becomes easy, transpose with eyes closed and you will be amazed at the results!
You can further strengthen the ear by eliminating muscle memory temporarily: for example, play the melody by alternating hands on each note (one note in left, the next in right, and so on). This way there is no muscle memory working for you and you are relying totally on your ear. Or play the passage in question with the opposite hand than normally plays it (you will probably need to go slowly). These methods may seem odd at first and you may be wondering why you would purposely make the playing more difficult than it already is. I can tell you, however, that I have never had a student who didn't become a believer in these methods after trying them and noticing an almost immediate difference.
I find that when the auditory memory is strong, I can still play the piece after months, sometimes even years, of not practicing it.
The other advantage of ear memory is this: if in performance you have a momentary slip, muscle memory alone may not be able to save you, but with auditory memory you can always find your way and get back on track.
Visual memory comes in two forms: picturing the written page in your mind, and/or picturing the notes on the keyboard. I think both of these will hold you back and prevent you from playing your best. People who are strong visual learners (and often good sight-readers) depend on the cues from the written page and may picture it in their minds when playing from memory. People who have photographic-type memories may not be able to prevent themselves from seeing the page in their minds. But remember, the written notation is only a vehicle for learning the music, it is not the music itself. If the visual cortex and visual processing parts of your brain are most active, your auditory cortex will have to take a back seat, and that means you can't be listening as intently as if the auditory is pre-eminent.
If you have memorized the piece to the extent that you are no longer looking at the page, nor picturing it in your mind, but you are looking at the keys themselves and needing that to give you visual cues as to what notes to play next, you will never memorize with ease or assurance. And this method is simply too slow for any fast piece. It is also not always possible to watch both hands at once, if they are spread out over the keyboard. If you need to look at your hands, the muscle memory is weak. The cure for this is, once you no longer need the page, practice with eyes closed. If you miss notes, don't immediately open your eyes and look; instead, use your ear to find your way back on track. Make it a goal to play the entire piece with eyes closed, even if a bit slower than normal. It is well-known that if you want to strengthen one sense, take away the others temporarily. Thus, to strengthen muscle and ear memory, take away the visual. Then when you do have your eyes open, it will just give you a last bit of security for things such as large leaps.
The last kind of memory, intellectual memory, is where you would be able to describe what is happening in the piece, for example, harmonically and structurally. This method, even more so than the visual, is too slow for any "real time" playing. You simply cannot think and process this information as fast as your hands and arms need to go! And again, if this area of the brain is too active, and there is a lot of "chatter" going on in the brain about the music, you can't really be listening and responding to what you hear. However, this type of memory is useful, and in fact necessary, for situations such as when a part of a piece is repeated, one time leading to one section but another time leading to a different section. Both muscle memory and ear memory know both versions, so intellectual memory is needed to remind yourself which of the two repetitions you are on, and where you are headed. Many a musician has suddenly found themselves in the wrong place in the piece because they were on auto-pilot, so to speak, and lost track of which part of the piece they were in, by depending only on muscle memory.
The danger with intellectual memory, however, is that it can get it the way. The last thing you want, especially in performance, is to start "thinking" about the notes, about what note comes next. If you do, you almost surely will have a memory slip. This is because thinking interrupts the muscle and ear memory, which, hopefully, at this point are secure. You must learn to trust your body and your ear, and stay out of the way with your "controlling mind" except for a high level awareness of where you are in the piece and where you are going. Then you are free to really listen, play from your heart, respond to what you hear, and enjoy the process!
The final reason that people have trouble memorizing a piece is that they wait too long to do it. They play the piece for too long still using the written page, and then leave memorizing all for the end. You need to be memorizing as you go. I often ask people to play 4 measures of a new piece, maybe two or three times, and then memorize it right then and there. Quite often, they can do it, much to their surprise. Now, I'm not saying you should memorize a piece 4 measures at a time -- that would chop it up too much and give you an un-musical result. But you can be memorizing elements such as the melody plus the bass line, or a simplified rendering of the basic harmonic progression, very early on in the process. Get the music "into your ear" with the methods outlined above, and you may find you know the piece by memory much sooner that you thought.
Being able to sit down and play a variety of music, for yourself and for others, by memory, is a joy and a wonderful feeling of freedom. To my mind, playing with the written sheet music in front of you can never be quite the same experience. If you've been concerned or fearful about memory up until now, I hope you will try my suggestions.