Well there are basically two things you can do to improve your downhill times. The first is pretty obvious--know the course. If you know where the crazy spots are, you can finesse your way through them, know where you ought to jump, etc. I've noticed a huge difference in the way I ride all the local trails now compared to the first time I rode them. The second thing you can do is just increase your fitness level and relexes, and get the best downhill form possible, for me that's leaning so far back over the rear wheel and bashing my way over most things. There was a really excellent article in a recent issue of Bicycle magazine, you may find it online. The gist of it is to push your personal envelople, also, try and use your perihperal vision rather than focusing on the one obstacle that scares you, this actually somehow boosts your reaction time. Of course, good muscle tone also increases reflexes. I think what has helped my speed is focusing on the inside of the curve, regardless of whether it's tight signle track or USFS roads at speed. If you look at the inside of the curve, where you want to go, you'll hold the line a hell of a lot better than if you're looking down a big root you're trying to avoid. If you want to avoid something, look at where you want to go, not where you don't. If you're watching your buddies tire in front of, you're missing out. Remember the old phrase, "keep your eye on the ball?" Well for us mountain bikers it's keep your eye on the line! Look at least 10 to 15 feet in front of you. Don't forget to keep your front wheel weighted and use your brakes when they'll work, not on the soft stuff that'll skid. Use your front more. Turns at speed are scary but are a big place to keep or lose your speed. Also practice your technical skills, like bunny hopping to clear rocks/logs where you feel comfortable, so that when you really need them, they're instinctive. Another trick I learned when riding with better riders than me was to retry sections that you can't clean, uphill or down. This helps more than anything to build confidence and technical skills. Trying any of these should increase your "comfort zone" on the dodgy downhills.