I have been cycling with a pack of women lately and was relieved to find out that I was not the only adult afraid of riding downhill. Although my 5-year-old hates the small build up of speed on the decline, I knew there was joy to be found in racing down hills.
The first thing to learning to love riding downhill is to simply ride downhill; you have to get comfortable to the speed of the descent by actually riding downhill. Learning to feather your brakes – applying the brakes gently and slowly increasing the pressure, then releasing the pressure a bit and repeating until you are at a speed you're comfortable with – will certainly increase your comfort level too. Of course, you need to apply both brakes simultaneously. Also, one simple way to slow yourself down is to sit up; your body will create resistance and naturally decrease your speed.
For another simple tip
You also want to shift your weight back and not put too much weight into your handlebars, a common mistake. Don't forget to keep scanning the road in front of you for potholes and other obstacles that could disrupt your ride (I know that sounds a little euphemistic, but since we are learning to love the act of riding downhill there are certain words I want to avoid).
What truly shifted my fear of riding downhill into love was my competitive nature. After working hard to passing people on the incline, I would be passed by speed demons on the descent. This just killed me and pushed me to give into gravity and love riding downhill.
Fit's Tip: If you still feel shaking after practicing descents, try getting your wheels "trued." If your wheels are off, they will make your bike feel more wobbly. A sensation no one wants when speeding downhill.