Being able to break down techniques or phrases into smaller, easier blocks is essential to really mastering something. To that end, you have to be able to approach the whole process very rationally. It’s pretty much disaster to work on more than one thing at a time. I’m going to use an example from a piece I’m working on.
Here’s a hard bit from Leo Brouwer’s Preludios Epigrammaticos no. 2.
There’s a lot of stuff going on here. Above is says Agitato, the whole thing crescendos. For now, let’s leave the musical instruction alone. The first step to working out a hard portion of music is wrote in your fingers if they are not already there. Doing this allows you to practice the part the same way each time, reinforcing a single muscle-memory pattern. I have a “system” for this, but anything really works. I write the RH fingering above the staff and the left hand below. Here’s the fingering:
If you play through that really quick, you’ll notice that there’s a huge shift between the two groups of 16th notes. The hand has to go from a more outwardly rotated position to a more straight position and shifting up a fret for the second half. This is a problem, and that shift is the hardest part.
To solve the problem, I practice the things on each side individually until they were very solid. Then I used stop/go practice to specifically nail the shift. I play the first part through at tempo, then stop, shift to the new position, hover then place and play at the same time. This finishes the run at tempo. Essentially it’s like playing at concert tempo with a little hiccup in the middle. When that’s down, the next step is to stop after the first run, wait, then move, place and play at the same time. After that’s mastered it can be tried at concert tempo.
This is just one example. Real critical thing about the physical movements behind a piece can be a powerful tool. Just being aware of your arm moving from a rotated to knuckles parallel position can have an amazing benefit–if you notice something, you can practice it! Beware of letting your hands go on auto pilot in the beginning stages of learning something. Think carefully about how it works, and practice carefully.