Ditto, I don't think most people can do the head between their knees thing with a seat in front.

The more common brace position is to put your head between your forearms (superman style) as close as possible to the seat in front - so you are a) looking down, to prevent whiplash, b) the closer your head is to the solid object in front of it, the less damage will be done on impact, and c) you have some protection for your head in the forms of your arms. Some airlines show hands over heads to protect the back of your head, and I prefer this position - with my forearms on the seat in front, palms on the back of my head (not linked) so there is a) some soft stuff between my skull and the seat (forearms / wrists are expendable), b) it keeps my arms tight in, to avoid them flailing and getting broken and c) the most likely place for things to land is on my head, again, my hands are more expendable than my skull.