go up, then down some moderately large, steep hills. do more and more hills until you feel more comfortable going faster.

when you're braking going downhill, at speed, try to stay away from using the rear brake too much, and try to train yourself to not grab a huge handful of rear brake in a panic situation. at speed, around a corner, if you grab that rear brake too hard, down you'll go. if i really need to control my speed on a descent, i hold the front down for like, 15 seconds, then let up for 5, then hold the rear for 10 if it's straight, then let off for 5, and go back to the front. that helps the rims and pads cool down...or maybe it doesn't and i'm totally out of my mind, but it makes me feel safer. and when i say "hold" the brakes, i mean very, VERY lightly, especially in the rear.

i used to be (and still am to some degree) scared of descending, but the more i do it, the more confidence i have. i always take it easy on a hill i don't know very well. get out there and ride. the more you do it, the easier it'll be.