I'd have thought it would be the last thing on your mind, but what I would probably do in the circumstance is:-
1) Avoid hitting objects in the first place
2) If 1) seems inevitable, try not to hit it quite so hard, and minise the impact
3) If effort 2) fails, give in and grab a cushion (if there is any) to protect the head and vital organs
4) Crack the door open so that it's less likely that I will get trapped in the wreckage
5) Get the headset and sunglasses off to minimise injury and being trapped by the headset cord
All assuming there is time to do so, of course.
I am not sure whether to tense up or loosen up, but to be honest, I think I'd be rigid with fear and will not have much choice in the matter...