When we learn a piece of music we imprint patterns of specific sequences of physical actions, which are tied to auditory and visual feedback. Each time we come back to that piece of music, we access memory of those sequences and the feedback we associate with them. Simply put, we move our hands and can judge our results based on how things sound and look.
In the earliest stages of learning a piece of music, it is very important to be very careful about those things that we imprint. If we can make those first steps as close as possible to the final intended result, we will imprint physical, auditory, and visual memories that will help us play more accurately. The easiest way to help insure this is to practice very slowly so that attention may be given to all of the little tiny details. It is also very helpful during practice to give attention to all three systems of memory/feedback: the kinesthetic (or physical), the auditory, and the visual.
When we play from memory, we access the imprinted information of these three systems. Most guitarists seem to favor one of these systems over the other two, focusing attention almost solely on it while playing. If there is a lapse in that dominant system, and there are not easily accessible and strong patterning in one of the other two systems, a memory slip or technical error occurs. There is redundancy, and if one system fails, the others can help. I think of it as a three-legged stool. If one of the three legs is weaker than the other two, then I will fall in that direction.
Think of a piece of music that you play from memory. What is the first memory you access? Is it how your hands move to execute it? Is it how it sounds? Or is it how the notes look on the page or how your hands look while playing it? Can you imagine the entire piece from beginning to end, focusing on each of the three systems individually?
Find your dominant system, and then eliminate it in order to strengthen the other two. If you find that your physical sense of memory is dominant (as it is for most of us), practice reading through the score and hearing the music very clearly in your head without playing it. Singing is also very helpful. Close your eyes and see if you can visualize either the notes on the page or the shapes of your left hand all the way through the piece.
If you find that you rely heavily on your visual memory, whether it be looking at the music or your hands, play with your eyes closed or play in a dark room. Concentrate on really hearing what you play.
For most of us, the auditory memory and attention is usually the weakest. It was a depressing and incredible discovery for me to realize that I often play without really hearing myself. However, eliminating the auditory sense from playing can be very useful in strengthening visual and kinesthetic memory. Try completely detuning your guitar, and try to play a piece you know, or mute the strings by placing a cloth between them and the top of the guitar (this is also a good way to keep from disturbing people who might not appreciate hearing you practice). Try writing a piece of music on staff paper from memory. You might find that you often don’t really know the music that you thought was memorized.