I don't think I can give you any better advice than Oak, but I can commiserate.... I'm small and light. I can out climb lots of people, but I get easily caught again on the way down. I also sometimes have a hard time staying with the group on descents if I've climbed with them on the way up - at the very least I have to work crazy hard to go down as fast as the bigger folks, so the descents are no rest for me.

One piece of advice I might be able to give. I don't know about you, but I'm small and short with short arms. I've found that though my bicycle fits for all the other uses, my weight is not really ideally distributed for going down hill. If I scoot my butt way back - mt biker style, so that much of it is off the saddle and more over the rear wheel, I feel a lot more stable through fast corners. Now this of course doesn't help if I have to pedal, as in that position pedaling is not possible, but it does help when just freewheeling.