Another downhill spin tactic: rather than letting the pedals pull your feet, keep on putting on pressure to force the spin- try to focus on the back side of the pedal stroke, as that's where people tend to get jerky.
By keeping the spin smooth, you feel more in control, and you're not jerking around on the bike, which can be scary on fast descents.

To keep your speed under control, sit up straight for air-braking action, and use the brake as required.