Consciously work at it. There is nothing worse for your hands and wrists than the downhill death grip. Not only does this lead to severe and eventually chronic pain, tensing your hands gives the subconscious the go-ahead to tense everything else along with them. Remember, it takes a lot of energy to ride downhill. Keep as much in reserve as you can, particularly on long descents.
Exhaustion is fear's greatest ally.
Take deep breaths. We sometimes even sing to stay calm. After asking around, we discovered that almost everyone we know has a favorite downhill song. Aside from serving as a minor distraction, singing or whistling requires you to breathe. You don't have to sing as long as you keep breathing, and remember that breathing out is just as important as breathing in.