Ultimate Sleds?

frenchgates asks: "I recently moved near a long and steep sledding hill with dangerous trees right at the bottom. I have acquired a fleet of sleds at this point, plastic shells, inflatables, a toboggin, a flexible flyer, etc, but all have one potentially fatal (literally) flaw: no brakes. I figure this crowd probably takes its gravity-fun seriously so I am looking for info on sleds (or customizations) allow rapid stopping in all snow conditions."