When woodshedding difficult passages, it can be helpful to change aspects of the actual music, for practice purposes. Remember these four variables mnemonically with GRAM: Groupings, Rhythms, Articulations, and Metronome.
The human brain is very good at organizing stuff. Babies put the blocks in one pile, stuffed animals in another. We do the same thing with music. We look for recognizable patterns and immediately implement the patterns into our thought process. We see some notes from a scale, a skip upward, a set of three notes together, etc. Practice Monster gets easily frustrated when we think too hard . . . but changing things up distracts him, thusly prolonging a successful practice session.
As an analogy, imagine that we were practicing Mary Had A Little Lamb. The first groupings are:
MA-RY / HAD A / LI-TTLE LAMB
Recite the words as they are grouped: two syllables, two syllables, three syllables. Now, imagine that we changed the grouping to 3, 2, and 2:
MA-RY HAD / A LI- / -TTLE LAMB
If you play an instrument, finger the notes as you recite the syllables, with appropriate breaks. It will feel awkward at first, because the grouping is purposefully unnatural. This exercise provides an opportunity to play the same sequence of notes, but with the mind grasping them in a different way. Imagine playing the Prelude of the first Bach cello suite, but two notes at a time, and then in triple groupings. The sound and feeling would be totally different, even though the sequence is the same.
I have found this technique particularly helpful in learning long strings of sixteenth notes. By breaking the notes up into different groupings, and inserting rests to separate them, we divide a meal into manageable little bites.
Grouping variations can be practiced out of time (rubato), or with the metronome. I like to start without a steady beat, and then introduce rigid time. By practicing a variety of groupings, the mental process becomes more supple. When we return to the original version of the excerpt, there should be a better sense of flow.
Stay tuned for more on GRAM Variations.