I came relatively late to classical guitar and score reading, therefore, I've always played (since I was twelve) from memory. I cannot explain it properly, but memorizing a piece implies a combination of images of the fretboard, a mechanization of the intervening fingers, chord positions and passages between them (trying to use as much as possible chord positions I'm familiar with to cover as many measures as possible), the whole melody slowly building "in your head". Having a piece memorized does not imply that I will play it well. Mistakes will happen - specially when you try to increase the tempo - but I will know immediately what I did wrong, not needing to consult the score.
As I am a very poor sight reader, I try to memorize the piece phrase by phrase (one or two staff lines, generally), by playing the first phrase over and over again until it is "in my fingers". Then I extend the practice to the following phrase, but starting again with the first phrase, until this second phrase is also "in my fingers". T this stage I am already playing a chunk of the piece without even knowing how it finishes
. I then extend the practice to the following phrase and so on... In order not to get tired of doing always the same thing, I intersperse this practice with other pieces I'm already familiar with. This gives me a degree of comfort and confidence and "touching" these other pieces prevents me form forgetting them as well. At the end of the day, I'll have logged much more time with the new piece than with any of the others I already know.
Using this "method" can take me days, even weeks, to memorize any given piece. As an example, Sor's Opus 60 #24 took me a week to memorize. I could then play it at a slow, comfortable pace, without too many mistakes. I put then a metronome at work to make sure that I could play it in a fluid manner - I'm now playing it at 1/4 = 86 bpm and I'll slowly increase the tempo until I reach the one stated in the score (Allegro moderato
, 1/4 = 112-115 bpm, tough I find this too fast).
Having said all the above, I wish I could also play by looking at a score. You see, at least in my case, when I play a piece (by memory) and make a mistake, because the whole thing is mechanized, I find it almost impossible to ignore it and proceed, my tendency is to go back a bit (one or two measures) and start again from there. It's OK if you are playing for yourself but not OK if you are playing for an audience.