The descent is your reward for the effort it takes to climb that hill or mountain, so learning the essential skills that will bring you down safely is an important rite of passage for a competent cyclist.
The art of descending is all about skill, perceived then practiced. Let's leave courage out of the equation for now because that implies taking risks as opposed to calculating them. It is to the subject of learned skills and the preservation of life and limb that I dedicate a few thoughts about cheating gravity and getting away with it.
Any discussion of the techniques involved in descending must start with getting to know the dynamics of a particular descent. Many of the best Tour riders scout the difficult sections of a race in advance, because it gives them an idea of what must be negotiated ahead of time. As you descend, you must feel the bike and connect with it so that you are a single unit. Get in tune with the pitch of the hill, the camber, and the way it banks, then climb back up that hill and ride it again. Skill and confidence improve with familiarity.
Also consider changing weather in the mountains, the potential for road hazards, sand, gravel, dirt, leaves, and freshly fallen rocks on sharp road cuts. If you are venturing into unfamiliar terrain, assume that the worst-case scenario awaits, and avoid speeds that compromise your personal skill level. Descending requires 100 percent of your attention, so do not allow yourself to become distracted and never take your eyes off the road in front of you.
If you know a skilled and experienced cyclist who will ride with you and enjoys playing the gravity game, join him or her. Follow his lead, though not with blind abandon, and attempt to duplicate his moves downhill. Ask questions, and wait for answers. As a rule, the best line through a downhill turn is outside, inside, outside, but the quickest way down may be completely inappropriate in traffic.
The manner in which you are set up on your bike is also a factor. Triathletes on steeply angled, forward-positioned time trial bikes, need to reduce speed exponentially, as these bikes may compromise a safe descent.
A fun way to learn descending techniques is to carve cones (water bottles) placed in an empty parking lot. You can safely learn how to control and balance your bike on the flats, which will make you more adept at controlling it on descents. In case you are wondering, practicing these techniques on a mountain bike on loose surfaces translates well to a road bike. For example, sliding your weight back on the saddle, reducing your center of gravity, light feathering of the brakes, and adjusting the brake bias are skills common to both road and off-road riding.
A less common, although potentially terrifying experience for an inexperienced cyclist generally occurs when one's center of gravity is too high and the mechanical set up of the bike is not optimal. A very sudden loss of control, which is characterized by high-speed oscillation, can be corrected with a remarkably simple action: simply clamp the top tube with the knees. It also helps to drop your center of gravity by bending the elbows slightly and sliding to the rear of the saddle.
In case you think your discomfort with gravity is indicative of your cycling rank, consider the plight of a Gianni Bugno, who won two back-to-back world professional road-racing championships in the 90's. Supposedly, Gianni had difficulty with fast descents early in his professional career, but he resolved the problem by listening to classical music at a largo tempo while descending on his bike. The slow, melodic music had a calming effect. As you acquire your descending skills, don't just practice, learn to enjoy the thrill of a controlled, safe descent.