Downhill SteepsGoals: (1) control descent, (2) keep balance, (3) avoid an endo
Done by: (1) body behind seat, (2) feet level, (3) knees flexed, (4) weight off hands, (5) elbows slightly bent, (6) feather brakes
First, eyeball the slope, beginning to end. Pick your line from top to bottom. Where are the tire traps? Where will you unweight? Where will you need to lift the handlebars? Where are the spots that allow safe effective braking?
Make your decision about speed. How fast are you going to ride it? Will it be a go-in-slow and run-out-fast, or will you need to check your speed at the bottom for a turn or sand-pit? On most rough steep slopes, speed is your friend. It actually smooths out the line, allows you to bang through and over obstacles, and helps keep the bike upright. If you're creeping down a steep slope, even the tiniest bump is an invitation to endo. Riding a rough steep downhill slowly is really "trials riding" -- it requires much better bike control. For average-skill riders, the downhill philosophy is "you stop, you fall." Keep the bike moving, at a rate that smooths out the flow.
Approach the steep spot with a little pressure on both front and rear brakes. As you reach the steep area, put the pedals level and begin to stand up. As you raise off the seat, push the handlebars forward with your hands as you arch your body. Feel the seat move forward between your thighs. As the slope gets steeper, bend your knees to bring your hiney down and back, closer to the rear tire. Keep your weight on your feet, and keep your feet "flat" relative to gravity -- as though you were standing on a flat surface.
No matter how hairy the upcoming slope looks, you'll do better to "ride it out" than to try a dismount on a really steep spot. If you're losing your balance, let off the brakes while turning the front tire slightly towards the fall line. Regain your balance, then reapply the brakes.
As you're approaching the bottom of the slope, let off the front brake. As the bike picks up speed, get your body directly vertical (vertical to the earth, not 90 degrees to your bike). Make sure you're not "pulling back" on the handlebars. As the bike reaches the level ground, stay loose. Subtly help the handlebars rise towards you, and let the bike rotate under you. (If you keep your body back behind the seat as you transition from steep to flat, you won't be able to pull yourself up over the seat when the bike levels out.) Let the bike roll as you regain balance and control.
If the slope falls suddenly into a sand-pit or loose rubble transition, or has a sudden ledge before the flat area, you'll need to make an active move to level the bike. (If the front wheel hits the loose stuff with weight on it, it will dig in, and you'll endo.) About a bike-length above the transition, quickly brake a bit to check your speed. Then, as you let off the brakes (completely!), straighten up your body and pull the handlebars back with you. Your front wheel should leave the slope, and the bike will level as the rear wheel free-falls down the ramp. The goal is to have your body, and the bike, in a flat-earth riding position as you touch down in the slop.