Did this at Mammoth Mtn, California, on a "day off" from climbing. You take the gondola up the mountain and let gravity do most of the work to bring you down.
1) Gravity doesn't do ALL the work. You have to steer hard corners, deal with drop-offs, navigate through narrow sections, and try to stay upright.
2) Wear a helmet that protects your face as well as the top of your head.
3) Be realistic about what difficulty of trail you pick for your descent.
On Mammoth, I over-estimated either my skill or my courage. I walked several obstacles, feeling like a complete slug, while teenagers flew by like kamikazes. I crashed twice, once spectacularly: I was tearing through the trees at about 30 mph (?-- it seemed about that fast! I truly was tearing) when my front tire hit a patch of deep volcanic ash; the rear tire kept going, hurling me over the handlebars. I creased a few trees and landed like a sack of bricks. Well, I don't actually remember landing, only that geting up hurt a lot, and I was bleeding from 'way more places than the previous week of climbing rocks and mountains had caused. OK, I admit it, it was tons of fun. When I got down, I went up again. Most of the blood had dried.
4) Get a rental bike for this kind of work; it's a lot of wear and tear.
5) It tends not to be cheap (lots more than a Preserve day pass), but it's worth it (like a Preserve day pass).