Tbh, octave technique is best learnt for yourself. At the end of the day, I think you've got to 'discover' the feeling for yourself. At the beginning, you may have a stiff wrist, a bit of a stiff arm etc, but after using the movement, along with thinking about the, very good, advice that has already been given in this topic, you should develop the necessary muscles to cope with octaves and begin to really get the movement from the whole arm. If you can't do it to begin with, don't worry at all. It will come with time.
I played La Campanella by Liszt recently, which has a tricky repeated chords passage at the end. I was ok with my right hand, but I have never really done this in the left and so my hand was reletively undeveloped in this respect. I tred to emulate the action in the right hand, but found it almost impossible because my right hand wrist was far stronger than my left hand (possibly from playing cello as well). So I simply tried playing the notes (slowly at first) just as my left hand would do them naturally. Slowly but surely, I locked on to the necessary action, and could eventually play the passage without a problem. Actually, my left wrist still isn't as quite strong as my right, but it's getting there. I thought it might be helpful to tell you that to describe more fully what I was trying to say to begin with. Anyway, good luck with the technique work.
Jon