The best thing you can do to increase your speed on a downhill is to gain a lot of weight. Start with a big breakfast with bacon, eggs, hash browns, and a stack of pancakes with butter and syrup. Drop off at the donut shop on the way to work and pick up a couple apple fritters. Find a sandwich joint that makes really big sandwiches for lunch, and remember to get some chips and a big brownie. And a 32 oz. soda. Have a couple candy bars at break time. Pizza for dinner, and a big slice of pie for dessert. More pizza as a midnight snack.
You'll have a hell of a time getting to the TOP of the descent, but you'll be unbeatable going down it.
Seriously, though - if the descent is long enough, spin out your biggest gear, seated and in the drops, and then tuck for all you're worth. Flatten your back enough that you start worrying about your jersey pockets acting like tiny parachutes. I also point my knees in toward the top tube. Never done supertuck, though. Generally I keep my hands in the drops on the brakes, since 40+mph can be scary when you're only wearing a few oz of lycra.
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"Everybody's gotta be somewhere." - Eccles