From my understanding, which may be far from the official story, you need to consider a few things. Minimize velocity of your head and neck, and if it's tight quarters and there is a good chance for a severe bend, make sure your head/neck bends down, not up (snaps your neck faster). I suspect that in many circumstances you may be better off getting up and hugging the seat in front of you. This of course is pure speculation.
I don't think brace position is given much attention from the airlines and authorities, most airlines that I regularly fly on don't even demonstrate the brace position (BA with their video, which is astonishingly useless). Installing proper seat belts would have happened long time ago if sudden stops occured with a certain frequency.
If I could get my head all the way down between my knees, I would never leave the house...
