In case you haven’t seen the Hash movie yet.
To set hair stiffness:
Find your Hair material, hit the plus sign, click the disclosure triangle for ‘Hair System’, turn on ‘Dynamic options’ and open up ‘Targeting’.
Angle limits:
Limits the maximum angle at which the hair can bend. For example, if you find the hair penetrates the skull too easily when the head is moved, you can tilt the hair upwards by say increasing the angle limit from 180 degrees to 90 degrees. This will increase the clearance between it and the surface.
Now, I don’t use collision detection with hair, never had much luck with it. I’ve found collision works well with other dynamic constraints, but not preventing your models hairstyle from penetrating it’s forehead, shoulders etc. That’s why I rely on angle limits to try and avoid some of these intersections on a shot by shot basis. It of course won’t always be successful, especially a very vigorous movement.