as for euan, i wouldn't know exactly what your problem is with octave speed... do you hit wrong notes when you try to speed it up? or you can't make your tempo any faster anymore?
this is purely technical. no illusions whatsoever...
anyways, i usually and primarily relate diffculty of a technical passage with my relaxation. there is tension in our muscles which we sometimes don't feel. and so we think that we're ok. but this tension which we are not aware of, IMO, causes us to play wrong notes. when we play a wrong note, we are probably reaching for the right note but due to tension, we move slower. but because we have to be in time, we automatically play a different note in order to catch up with the correct rhythm. (because tension made us slower, right?)
(but if relaxation is not the problem, then i would move on to my next possible solution.)
so here is how i would practice:
try practising everything slow and softly. when we do this, all our muscles are at their best relaxed mode! everything in your hands is ALIGNED well. get that feeling into your brain.
next, pump up the volume a little. then get that feeling of your relaxed muscles again. then pump up the volume for the 3rd time...
now, speed it up a little, but start with playing it softly again. the process is a notch higher everytime...
something like this:
Tempo 1+Volume 1
Tempo 1+Volume 2
Tempo 1+Volume 3
Tempo 2+Volume 1
Tempo 2+Volume 2
Tempo 2+Volume 3
ETC.
the impt thing here is that before speeding up, we make sure that our "relaxedness" is up for the speed we want. if we play too fast or too loud right away, our brain will remember the muscles movements, which were mostly wrong: unrelaxed, not aligned, forced, etc., thus, new bad habits for that passage.
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also try shifting the position of the wrist (upward and downward) once every few octaves. changing the position breaks the tension for a moment. just make sure to make up for the sound. the sound can change when position changes. i saw stanislav ioudanitch do this with that hungarian rhapsody.
if you've tried all that i've said, then i know nothing else.
- crazy