I'm kind of chicken on the downhills, too. I always feel like I'm holding back my friends, but they still ride with me!
But, I have learned some things, too. Others have said them, but I think the most important thing (other than a well-fitting bike) for me is to push my bottom back as far as feels comfortable. It is amazing how that seems to stabilize your balance on the bike on the descents. Also, if you "feather" your brakes, your hands won't get so tired.