- Repeat a phrase of a size that you can both remember and play
correctly at a reasonable tempo at least 10 times with minimal mistakes. (If you are making mistakes every go, then the size of the phrase you are practicing needs to be broken into smaller pieces.)
- Take a break. Practice something else, perhaps a different piece you are
trying to learn (some how it doesn’t work as well if your brain
knows you are still on the same piece, but practicing something else
is OK.). Spend enough time that you are thinking about the something
- Go back to step 1 and practice the same material
that you practiced there the first time.
- repeat this procedure 3 or 4 times in one day, and practice the same section the same way the next day. Practice the same phrase in this way 3 days in a row, and you will probably remember it forever.
You can use this same technique on multiple phrases and pieces on the same day to multiply your productiveness. As boredom sets in, try to think of different aspects of what you are doing that you want to fix: finger placement, phrasing, rhythm (sometimes you can use a metronome with this practice technique), consistent picking. I used to think about the names of the notes too, but that is not necessary.
Some times humming along with what your playing helps too, but watch out. Some performers forget not to hum in real performances so it is not always a great option. It does improve memory somehow though.
note: The break (step 2) is super important! without it, practicing something 30 times will get a similar result to practicing 10 times.
Also, as you gain success in this technique, you will need to connect the phrases you have memorized using the same technique.
In this answer I use the word phrase in a non-standard way to just be a segment of music. Actual phrases are actually best, but they are sometimes too difficult to memorize in a single go. Then you break up the phrase smaller. Other times you need to include more than a phrase, to connect the phrases as you learn more of the piece.
Avoid the temptation to play the whole piece at first. In the beginning of memorizing a piece, you waste time playing the whole piece when you cant remember a 10 digit telephone number. Save playing more of the piece for when you have more of it memorized. (It's not that you cant ever play the whole thing, just do not expect that to help your long term memory very much in comparison to this technique.)
A sidelong hint: I can sometimes trick myself into thinking of different passages of a piece as a different piece and thus multiplying my potential to cover allot of ground in a single day.
sidelong hint 2: I often practice the last phrase first, and work from the end to the beginning. This is because many will make the mistake of always practicing from the beginning of a piece and will start over when they make a mistake in the playing of the music. The mistake there is that they practice the beginning more than the end, which guarantees that the last thing they play, the last thing an audience hears and thus remembers best, is the part you practiced the least.