What skills and techniques are required to descending at high speeds while maintaining control of the bicycle? How does this change when the roads are wet or have gravel in them?
Whilst I come from a mountain bike background so my advice might be different from someone with a road background, I've found the following tips work well when I've been on a road bike in the mountains.
On wet roads, go slower :-) The same advice applies, but it's even more important to not use the front brake in the corner. And in gravel, take it slow - it's just not worth taking risks because you can lose the front wheel without any warning.
The best way to lean to descend mountain roads is to follow someone who knows the road and has carbon rims. You'll be able to hear them braking and you'll be surprised how much faster you can go!
The only thing I have to add to Chris's answer is that brakes can overheat rims on long descents. I'll either alternate brakes front-back where possible, or gently pulse them a little from time to time.
Another tip or two from a road biking perspective.
When cornering, remember to drive down the outside leg and lean on the inside drop. If you need to throw a knee out, that's fine, but keep your outside elbow in close and your head and shoulders level. This will help ensure your tires stay in contact with the road.
Like Chris said above, keep your eyes on the exit.
Just some other tips to add on...
Have a buddy with you when attempting a significant descent in a remote area.
Stay cool and try to mentally anticipate what is going to happen before you start reacting. Pick your line and think about contingencies if a car or obstacle shows up. Look far ahead, not just at your buddies in front.
Don't get aggressive with speeding up unless you've been on the road before and knows what lies ahead. Gravel is awful on a descent and you'll almost certainly wipe out on it if you try to take a turn on it at high speed. This also applies to wet roads to a lesser extent (although paint stripes are like ice).
Beware of "tar-and-chip" sections that can appear with little warning even on roads that have no need of maintenance. Rural municipalities are infuriatingly unpredictable about this type of maintenance.
If you just want to slow down a little bit (eg to put a little more distance between you and the guy in front), try sitting up or standing instead of applying the brakes.
As others have said, it is good to alternate front and rear brakes. I've heard of outright blow outs in the case of tandems (that's the worse case scenario). However, you'll notice that the brakes fade in power as they get too hot and this happens on any kind of bike.