I first heard the concept from the remarkable David Russell, who tossed out the idea in a master class long ago, along with myriad other indispensable bits of advice. This one proved both an extraordinary time-saver and a true confidence-builder. 

At first blush, the notion seems silly: why would you try and memorize music from the end?? We play, hear and understand music only as it unfolds normally in time, not in reverse order. How could it help to invert the music?  In fact, this method is remarkably effective. But the details of its application matter. 

It is important to understand that while this process is perfect for quick memorization, it is not intended for learning the music in the first place. That must be done in the normal sequence first, taking care to fully finger all the passages and resolve any questions of execution, whether technical or interpretive. The method described below follows that process, and will enable a fairly new piece to be quickly committed to memory. Here is the routine:

Commit the piece's final measure to memory. Now work on the penultimate measure. Always include with this measure the downbeat of the next measure, so that you are connecting them. It often works well to include a note (or two or three) from the previous measure as well, as the music is often phrased that way. (I prefer to memorize in chunks defined by musical phrasing rather than barlines, but most phrases will be longer than a measure, and small-sized units are more manageable. In long measures, it is sometimes better to proceed by half-measure units.)

Once the second-to-last measure is memorized, play both measures together from memory. Do it several times. Be sure to play with as much attention as possible to dynamics and other pertinent elements of expression. 

Now move to the third-to-last measure. Once again, include the downbeat of the next bar. Memorize it solidly. Repeat it until you're sure. Now play from there to the end. Several times. With each additional measure memorized, play to the end several times. This constant repetition of the already-memorized portions helps immensely to drive that memory in deeply. It is the element which guarantees success. But it is not the main reason this method works.

The reason it works is that you are building on the psychological comfort of playing towards material you know already. As this process unfolds, each new measure added has had slightly less repetition than the one memorized before it. Therefore, in normal playing, with each passing measure, you become more certain of what's next. You become more confident, more secure, more precise, more relaxed.

Now contrast this experience with memorizing from the beginning, in the normal sequence. As that process unfolds, and you start at the beginning, with each passing measure you become more and more unsure, until, at the newest measures, the ones just added, it finally breaks down. It's like driving towards the cliff at night. You know you're going to go over the edge but you're not sure when. You feel increasing levels of trepidation with each passing moment until the inevitable happens. When we build into our experience of the music this type of unease, it is only natural to have problems. 

Playing towards material you already know is so different from that it is hard to grasp if you've never tried it. You feel better, stronger, more confident as the piece unfolds. Generally speaking, most people learn the openings of pieces first. So if you memorize from the end, you will eventually bump into the beginning section which may be already memorized  or close to it. Using this method, your pieces will both start strong and end strong. 

In longer works or prices with multiple sections, you can treat each section seperately for purposes of memorization, though it's still best to start at the end. Therefore, in the case of a Bach dance suite movement, you will play to the end of the B section with each new measure added until the entire B section is memorized, but start the process over with the A section. This will save time, cutting out additional repetitions of the B section while working on the A section. 

I have found it manageable to memorize  entire Bach movements in one sitting with this method. Of course you'll need to repeat the music at your next sitting, maybe brushing up on some spots. But pay attention to it closely in the next few days and it should stick. Not only will it be well-memorized, but you'll have that uncanny sensation of feeling the music get easier as it unfolds. Of feeling your confidence increase with each passing bar. 

There are other terrific strategies for memorizing. The most interesting may be visualization. Visualization is hearing your music with your eyes closed and the guitar in the case, while seeing your hands execute all their respective movements in real-time. As you revisit your piece in this way, work to "see" your left hand make all of its shifts, shape-changes, and fingerings. Then do it again and "watch" your right hand. If your memory blanks, check the score and try again, working towards longer and longer excerpts, until you can visualize the entire piece. 

I find, with this method, and with the one described earlier, it is vital to include in your work both interpretation and general emotional temperature; in other words, even in the case of small units of the score like a single measure, it is important to play with appropriate articulation, dynamics, color and phrasing. The music should already include the right level of calm or intensity, of pathos or rigor, of lyricism or rhythmicity. Imbuing each and every phrase with emotional meaning, with rich and imaginative story-telling, will help guarantee a long-lasting and sturdy result when memorizing.