Don't look down, look ahead to where you want to go. Looking down at the pavement increases the sense of speed.
One thing I learned from riding a motorcycle at the track (admittedly not everything carries over) has to do with whether the vanishing point of the road is coming towards you or moving away from you. The road isn't actually moving, it is a matter of perception and that perception varies as you progress and become faster. If you feel like the road ahead is being "fired" at you, slow down, you are going faster than you are comfortable with and are probably going faster than you can react to. If you feel like the road ahead is "unfolding" in front of you, you can go faster than you are going.
Finally, words of wisdom from my sister who is madness maddened on descents: "15 miles an hour, 50 miles an hour, either way, its gonna hurt!"
So go fast!