Lots of good advice in this thread. I remember when I first started on the road bike and I didn't trust my handling skills going 35mph or more on a descent. The slightly slower twisty ones were a lot of fun, though. Go figure. I felt like I had a good knack for finding the right line in turns. The best thing is what you've started to discover: you need to relax your body so that your body can absorb any shock from the road. Some of the other riders you see may have tires that are grip better in corners, they may be better at countersteering and fine-tuning their weight distribution, or they may just be daredevils on descents. The next best rule is to never brake while you're turning. Brake when you can hold a straight line before the turn, and then if anything accelerate out of the turn. Just like with cars or motorcycles, this will help you stick. The more open the road (no blind corners), the more you can see far enough to really use the width of the road to your advantage, because you can tell if someone is coming at you, and you can plan your full line easier.

I also prefer to descend in the drops, but if I need to slow down and don't want to touch the brakes, I'll lean back, taking some weight off the bars, stick my elbows out some and try to make a parachute out of my body. It doesn't work as well as sitting up, but it's safer to me than moving all over the place or hitting the brakes.

As for your wreck, it sounds like you hit the brakes way too hard in panic. Feather them unless you're going dead straight through a pretty dry patch. Even then, don't grab hard all at once. If you're in a situation where you have to grab a ton of brake in those situations, you might be headed for a crash anyway, but I'd personally be in favor of doing what you know works about cornering (weight placement and countersteering) and hope the tires stick.