With rapid octaves I have always found it much easier to use as much finger movement (3,4,5) as possible, let the thumb take care of itself, keep loose, play close to the keys and keep wrist and arm bouncing to a minimum. This might only work for people with reasonably large hands though. It's rather hard to describe how it feels (looks like a spider galloping) and my technique has lots of flaws at the best of times. However, I can keep playing fast octaves indefinitely this way while I certainly can't if I bounce around too much.