Not sure if it was already mentioned, but try keeping one knee on the top tube, or clamp it between both knees, to keep the bike from shimmying.

I happen to love fast descents, but when I'm on one I don't know well and can't see exactly what the road is doing I have to remind myself to relax my upper body and keep my elbows loose and bent. I'll also gently scrub off some speed by applying light pressure on and off with the brakes.