This is what I was going to post. The best bike handlers and descenders in the pro road ranks generally have one thing in common: a background in mountain biking or cross.

Learning how to mountain bike has really helped my bike handling skills on the road. I started mountain biking about four years ago and it's now very noticeable to me how much better and more confident a bike handler I am. Learning how to lean the bike but keep your body upright, learning how to weight the bars properly (this was counter-intuitive for me), learning how to "body English" your way through a turn, learning how to brake properly before a corner and accelerate through it, learning how to keep your center of gravity both low and equally between the wheels, learning how to detect the limits of traction and deal with a skid etc: all skills that translate from mountain biking to the road. In particular, I've found a hardtail will force you to really learn these lessons. If you want to become a better road cyclist, buy a hardtail and hit the trails.

Note: I don't race cross, but I'll bet cross has a similar impact.