If the horse starts acting up, I put a stop to it by disengaging the hind quarters, getting their head down, and in a tight circle. Never, do I have the need to hold onto anything to let the horse freak out, I just simply put a stop to it before anything really happens.
As far as rearing though, let the horse have it's head, lean forward and grab some mane. You never want to even have pressure on the reins on a rearing horse. Pulls them right over backwards if you do so.
"You're just as sane as I am."~Luna Lovegood.