Erh, jamawani, great advice, just beware:
If you keep your hands as close as possible on the handle bar, you are sacrificing control. If you happen to hit a rock on the road or such, it will throw your tire off and since your hands are gripping in compromising situation, you tend to lose control.
Also, my advice is we often fear the unknown. Learn more about the downhill ride, and how your bicycle operates at fast speeds. If you are going so fast, the air flow starts to take control of your bicycle, I would recommend slowing down a bit. While I say this, you also have to take in consideration of break ware, and try and break as little as possible while riding downhill, but not so little that you lose control. I don't know how big of hills your taking, but when you grow up to mountains, watch out for hot rims. Mountains have long downhill runs, and your going to have to break during some of it to keep control and make the winding turns. Your rims get hot from the friction, and, if it's a really bad mountain, they can get hot enough to pop your inner tube, which, at 40+ mph, is a really bad thing.
Otherwise, follow jamawani's advice, and be carefull. Safe flying, my friend.