There is a lot of good physical descriptions of how to practice octaves, and how they feel when you get them going good. I want to add though for the sake of those who find those descriptions more disastrous than helpful, my own approach!
I found in my own playing that the biggest barrier in playing octaves was losing sight of the melodic content. I tried to play them bouncing, I tried to play them with wrist, in, out, zig zags, around, all that. In the end it didn't work for me. It was the natural breathing space of the melody that I was missing. On the recordings it is easy to be blown away by the octaves, and it sounds like exact, precise repetitions of movement. But it is not that, it is superior melodic playing, that covers up all the seams in how the movement actually takes place.
The way I discovered to practice octaves is to play them in single notes and really learn in depth the melodic contours, and all the gradations of sound within the melody, and sometimes I learn them as if they were compound lines like in Bach's works. This way the ear is naturally learning when to let up, and when to get more intense. The problem with a lot of people's octaves is that they want them to be equal intensity, all the time. Simply put, nothing works like that. After you learn the contour, when does it get heavy, when does it get light, octaves become, I guarantee, at least 75% easier. Maybe even more but that much I can guarantee. I play the octaves in Liszt Sonata a tempo without any wrong notes, whereas before, trying all these physical voodooisms, it was just a big mess. I hope that helps someone out there.
Walter Ramsey