There's a number of websites talking about descending mountains on a bicycle. Some of them are contradictory. After a nice morning of enjoyable climbing, it's important to get the descent over with safely. Maybe you can even enjoy the speed. Here's what I do to get down safely (not going for maximum speed).

Practice descending skills before you need them.

Brake judiciously: if the brakes and rims heat up, you'll get brake fade and lose braking power. If traction is good, you can use the front brake more. If traction is bad, favor the rear brake. You may be able to recover from a rear-wheel skid; front-wheel skids are difficult to recover from. On long descents (several thousand feet over several miles), I will sometimes put on my cycling jacket, sit up straighter, and open the front to let it act as a parachute to slow me down.

Motorcyclists do an exaggerated lean and put their inside knee out. DO NOT DO THIS! Motorcycles have a lot more rubber on the road than a bike. Keep your weight on your OUTSIDE foot and tilt the bike under you. Keep your weight above the contact patch of the wheels, not in-line with the bike frame. The bike can feel a little twitchy in this position, practice it before you need it.

Keep your knees and arms bent to help absorb unevenness in the road so your torso and head remain steady.

It’s great if you can know the course in advance. If you know that it flattens out after a given curve, for example, you can plan to come out of the curve a little faster. If you know that there's sand all over the road, you can not go up there in the first place!

There are other websites with a fuller explanation of the techniques; the two tips that I've not seen elsewhere are the jacket-parachute and knowing-the-course. Practice and be safe!