To memorize piano music, simply use the rules for learning, with the added proviso that you memorize everything before you start to practice the piece. It is the repetition during practice, from memory, that automatically implants the memory with little additional effort compared to the effort needed to learn the piece. One important element is HS memorization. When you memorize something beyond a certain point, you will almost never forget it. HS play is also the main element in memory maintenance. For memorizing, you can use hand memory, photographic memory, keyboard memory/mental play, music memory, and music theory. The human memory function is associative and a good memorizer is good at finding associations and organizing them into an "understanding" of the subject matter. All these memory methods should culminate in a process called "playing in your mind" -- you can play the music, and hear it, as if you have a piano in your head. You should have two repertoires; a memorized one, and another for sight reading. Memorizing is necessary for learning a piece quickly and well, playing musically, acquiring difficult technique, etc. Mental play brings with it a whole new wonderful world of musical capabilities such as playing a piece from anywhere in the middle, learning perfect pitch, composing, performing without flubs, etc. Many of those miraculous feats that Mozart is fabled to have performed are actually within reach of most of us!