How to descend mountain roads? 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.


Keep your weight centered - don't be tempted to lean back. You want to keep a good amount of weight over the front wheel to help it grip.
Look through the corner, not at the corner, focusing on the exit.
On dry roads with a good surface you can probably lean the bike a lot further than you realise.
Brake before the corner, not in the corner. Braking in the corner will cause the bike to "stand up".
If you have to brake through the corner, only use the back brake.
If you can clearly see through the corner, use the full width of the road (but ONLY if you have good visibility of what is coming up the hill).


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!