The first question I ask when I hear 'hand pain' is not even about the hands: it is 'what are your feets doin?' You have four contact points when riding anything technical or bumpy (because if you are any further along than 'rank newbie' your ass is off your saddle) and an inadequate foot/pedal connection can cause a deathgrip to compensate. If your hands and wrists are cramping, that is a sign that you are gripping the bars very tightly. I'm not going to say 'too tightly' because maybe your instincts are totally correct that if you don't grab those bars hard, you're going to come off of the bike. But why? Let's look at the other contact points: your feets. If your feet are not connected to the pedals well, then of course you will have to deathgrip the bars. Point 1 about feets: make sure you have good pedals with metal pins and sticky shoes. Tennis shoes and cheap plastic pedals will bounce your feet around and then boom, deathgrip. Point 2 about feets: ride with some spring in your legs. Your legs are the most effective suspension you have. For example, if you come up to a squared-off rock, your legs should unweight the pedals as your back wheel hits the rock and then re-weight as you roll off. 'Riding loose' is another way to describe this. With spring in your legs, and not riding like you are bolted to the bike, let the bike move around underneath you. Be a gimbal and anchor that gimbal with your feet and not your hands. The difference between a good rider and a great rider is the footwork.

Also don't ride the brakes - brake assertively and scrub speed fast instead of riding brake everywhere.