But no more! There are several techniques of memorization that I would practically guarantee will enable you to memorize a piece so that you will not even have to worry about it. They are not quick-fixes, they require effort, but are well worth it.
Visualizations can aid you in remarkable ways. Basically, visualization can aid your knowledge of the piece you are playing, and also aid your memory of the physical steps required to play the piece.
One form of visualization (advocated by the pianist Walter Gieseking) is to study the score of the peice until you know it so well that you could sit down with a blank sheet of manuscript paper and write it out from memory. No simple undertaking! Gieseking even advised doing this before you ever try the piece out on your instrument, but that might be going a little too far. However, if you try doing this experiment with a simple piece like Lagrima, I think you will notice that you are seeing in your head what your fingers are doing, translating that into notes, and then writing it down. This is s sign thqt you are relying on muscle memory when you play a piece, and this kind of memorization is notoriously unreliable. (The good news is that this type of memory can be made infallible too; more on this later) That is, you don't really know the piece, you know the fingerings. One way to correct this is by using solfege.
Solfege just means saying the names of the notes, either with solfege-syllables (do, re mi...etc) or letter names (A,B,C,...etc). Learning to solfege a line makes you become aware of the notes you are playing *as* notes. Let's look at an example. What kind of music is the most difficult to memorize? I'd say unending strings of sixteenth notes, as you find in some of Bach's music. The lack of rhythmic variety makes it hard to get your bearings. Take any Allemande, for example, from one of the solo violin or 'cello suites. Say the names of the notes as you sing it. Now play it and also say the names of the notes as you sing along with yourself. You are now conscious of the notes as notes outside of their finger-positions. Maybe you could even write the piece down from memory!
Being able to write a piece down from memory would certainly take you a long way towards memorization, but not all the way. Unlike the piano, there are too many different ways to play a given passage on the guitar, so memory of fingerings is important too. Besides, you can solidify your finger-memory in alot less time than it takes to study a piece so you could write it out. There are several ways to do this.
Probably the hardest is this: sit down in front of your music without your guitar and look at the first note or chord. Now, in your head, see clearly where this note or chord is played on the fingerboard, and what your fingers look like while playing it/them. (When I speak of seeing you fingers, I am speaking of the left hand. I personally have never found it necessary to worry about the right hand's "memory". If my left hand can find the notes, my right hand knows what strings to play.) Now do the same with the next note, etc. If you can get through the piece this way your memory will be much improved. (I remember the first time I did this with the Chaconne. By the end I had a headache from the intense concentration!). But even this is not the ultimate: when you get into bed tonight, pick a piece from your repertoire and play it in your head. Can you see clearly what your left hand fingers are doing at all times during the piece? If there is a passage where you cannot quite see them clearly, I would bet anything that this is where you will have a memory slip if you play the piece under pressure.
So how do you fix these hazy spots? Sit with your guitar, but don't touch. Look at the fingerboard just as if you were going to play, and *project* the image of your fingers onto the fingerboard where they belong. If you get stuck, bring your hand up and play just the part that you're stuck on, then drop your hand, back up and visualize again. This stuff works like a charm.
I've saved the best for last- the easiest and most effective technique of all - "Sequential visualization". Sit with your guitar and play the first note/chord of the piece, then freeze. Now, visualize the next note/chord and when you're sure you know where the notes are on the fingerboard and which fingers are going to play them, go ahead and put the fingers down and play. Then stop again and visualize the next link. Do this from beginning to end and I give you my unconditional money back guarentee - you will not have a memory slip in the piece even if someone is holding a gun to your head! What's more, this engraving-the-piece-into-your-memory lasts for months- even if you do it only once or twice on a given piece, the effects will last and last. And if after a year of not playing the piece you find that your memory-image of it has faded a little, a quick run through with visualization will get it back again.
There are more subtle and profound benefits of this type of memorization, especially if done in conjunction with solfege. You will find your sensitivity to the piece's nuances greatly increased, and the increase of confidence that comes from not having to worry about which-note-comes-next will allow you to give freer reign to your creative imagination.