One thing about going downhill, don't depend on your brakes to stop in case of an emergency if you've been riding them all the way down. Slow down and use the same gear you used going uphill, then the engine will do a lot of the braking.
Someone mentioned that the wind resistance will provide most of the braking force. That would apply only to the most gentle slopes. Many mountain roads can be in the 6 to 8% grades, and those require a heck of a lot more than the breeze whistling through your hair to slow a 20,000lb+ rig.
The main thing is to relax, take your time, and don't kill anyone.