Probably the hardest thing I had to learn is what Spokewench mentioned, the fact in mountain biking you can't mash you must learn to spin up a hill and use those low gears. This doesn't mean your cadence should be 80 or 90 like in road riding, but it is important to be consistant. When approaching a steep or technical hill, put the bike in low (not necessary lowest) gear and fine a easy pace. You might also want to pull up forward on the saddle, body bent toward the handlebars to put weight on the front wheel - helps to keep the wheel from lifting up off the ground. Also it depends on the type of bike you are riding - rigid, HT or fully. If on a fully, you definitely want to stay seated while climbing.

Descending is a skill and art. A fully takes alot of jolts out of the descent and you can stay seated more often. If riding a HT or rigid, you will be off the saddle, butt back over the rear wheel, torso level to the top bar for a lower center of gravity and using your legs as shocks. If on a fully, you still need to get the weight off the front wheel and allow the front shock to do its job. It's amazing all the stuff the wheel and bike will roll over. If the descent is steep, your body and weight should be off the saddle back over the rear tire.

I found mysef a steep but very short hill to practice technique. Descending is still not my favorite, I rather climb, but I'm getting better at it. It takes patience and experience and rememeber (yeah like its easy) never panic!