Along with some music of the 20th and 21st centuries, there's not much music more difficult to memorize than that of J.S. Bach. As a keyboardist, I write from experience, and I know that other instrumentalists and singers also find Bach to be among the most difficult music to memorize. I don't want to speculate too much on why this is, but, as a case in point, I can honestly write that it takes me about twice as long to memorize a Bach movement than it takes me to memorize, say, Handel or Couperin. And it easily takes me about four times as long to memorize a Bach fugue.
In conservatory, I was taught to memorize Bach fugues by memorizing each voice separately. I remember doing this. It was absolutely nuts. I also vaguely remember bringing two voices together, then three, in all manners of possibilities. This was a long and arduous process, and, today, I really question the musicality and practicality of this approach.
Today, I memorize Bach fugues just as I memorize other works. I use my landmarks, metronome, practice log, mental work away from the instrument--you know, all those good habits. This entire process is intensified by the fact that Bach, unlike so many contrapuntal composers, accounts for every single fugal voice with striking regularity. (Handel's fugues, in contrast, are much more informal, with dropped lines and voices that don't follow through from their starting points.) In this memorization process, one doesn't just memorize notes or chords. For Bach, each note is part of a line that is carried throughout the entire composition; each line is a living entity that is co-dependent upon the other voices in the fugue. That, my friends, is the highest level of contrapuntal writing!
When I performed Bach fugues from score, the learning process did not require me to know what each voice was doing. I intellectually understood that I should be aware of the voice leading, but, since I wasn't memorizing the music, really understanding and committing to the counterpoint wasn't essential. At that time, I played fugues with a certain level of indifference to the compositional process.
Memorization has forced a new respect for Bach's fugues. The number of voices has become paramount to the learning process, something that wasn't so much the case when working from score. Once memorized, there are always new discoveries in the relationships between the voices. This can be a distraction on stage, especially when hearing something differently for the first time...in front of a live audience! For this and for so many other reasons, I consider Bach fugues to be the hardest things to memorize and, once memorized, perform.
Aside from the techniques already described, there's not much advice I can give to learning fugues any quicker than the very slow route. I do advise, however, thinking of the appearance of fugue subjects and answers as landmarks and mnemonics. Along with larger phrases, fugal entrances can make good landmarks. At the very least, the performer should know the placement of each and every one.
Fugue subjects and answers can also be thought of as mnemonics behind the foreground of other material that's developing. In my earlier Wikipedia quote, mnemonics are used when "the human mind more easily remembers spatial, personal, surprising, physical, sexual, humorous, or otherwise 'relatable' information, rather than more abstract or impersonal forms of information." If we think of a fugue subject or answer as a spatial or physical element, we can concentrate more on the activity around it, thus aiding the memorization process.
I know, these are tough thoughts to consider. But, I assure you, memorizing Bach fugues is much harder! Good luck.
