Well, gadgetguru, you can pooh-pooh my suggestion all you want but from a purely physiological perspective, relaxing your limbs is the thing to do. Have you ever been in a helicopter crash? I have.
strapped to a gravity-succomed, power-plant-deficient rotary-craft, plummetting to earth with only moments of an uncertain variety of thoughts pummelling your conciousness; am i going to walk away from this, am I going to crawl away from this, am I ever going to walk again, am I going to make it out of here alive, to childhood memories, loved ones & who knows what else.
What kind of pathos is that? I can assure you that you will not have time to have all those thoughts 'pummeling' through your head as you go down. What kind of helicopter do you fly that is 'gravity-succomed'?
If you can learn to remember to lower the collective in case of an engine failure, then maybe you can try to remember this bit of information when a crash is imminent. Having said that, the issue is somewhat academic as most helicopters impact the ground at a high vertical rate rather than a horizontal rate, and learning to relax the sphincter muscle will not make any difference except perhaps to the color of your pants. Good crash-attenuating seats will help you more than anything else.