As mentioned in one of the previous posts, find a straight hill and start slow but increase the speed you ride down it each time.
FWIW, I had the same problem. Was scared silly of "Quarry Hill", a 1/2 mile 20+% gradient hill. Hit it the first time (mtn bike at the time) at about 25 mph, then 30, 32, 35, etc. I finally got to the point where I would make my cousin, who usually rode with me, stand at the corner before the hill and give me a wave when I could go all out before the TOP of the hill and have as much speed as possible. I now LOVE high speed descents, as long as they aren't too technical.