You've been there. You ride your dirt bike to the top of a hill and then lock your brakes up at the top when you see the steep drop on the downhill side. There's a trail... so other dirt bikes have gone down there. There are no skeletons wearing helmets at the bottom... so apparently they survived.
Whether you're dealing with a short, steep drop off, or a longer downhill ride, the technique is pretty much the same.
Most dirt bike riders, when confronted with a steep drop, will tend to drop to the seat because that feels the most secure. The first thing you have to do is get your butt off the seat. And, here's the part I love to say -- stay relaxed. Yes, you might be nervous, but tensing up will result in losing some control of your steering, braking, and balance.
Make sure you pick and start traveling at the speed you want before you begin to tackle the slope. In general, if you're feeling confident halfway down it's not a big deal to pick up speed. If you panic on the way down it's a lot harder to slow down and keep the dirt bike under you where it belongs. On your way down, you should be using about twice as much rear brake as front brake. Too much front brake and you're going to beat your dirt bike to the bottom.
Keep your body loose, stay up on the pegs in a crouched position.
As you would with any obstacle, figure where you're going to go at the end of it.