This article is from the Piano Playing from Memory FAQ, by Isako Hoshino rmmpfaq@yahoo.com with numerous contributions by others.
Once the memorizable units have been identified, you can proceed
one by one to master them and then integrate them into the whole
of what you have memorized so far.
The number of repetitions required to commit each musical unit to
long-term memory will vary for each individual and among the units
themselves. Do not try to proceed too fast, for newly forming
memories may be crowded out by subsequent ones if the former have
not yet solidified. If today you cannot remember what you worked
on yesterday, go back and work on those parts some more.
It is more effective to memorize in frequent brief practice
sessions than in fewer longer ones. Last minute prolonged cram
sessions work fine for remembering over the short term, but long-
term retention is poor. However, if you have four months to learn
a four movement sonata, it is better to acquire a shaky memory of
the whole thing the first month and improve your memory in the
remaining time than it is to learn a movement a month.
 
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