From the June Inov-8 Newsletter. Inov-8's have been my favorite trail shoes for several years due to their nimble trail feel climbing and descending.
"Guide to Trail Running: Technique
Trail running requires a better sense of running technique than running on the roads. Try to stay light on your feet as if you are running over eggshells.
Relaxing your body improves form and technique more than anything. Foot placement starts to come naturally with practice.
Resist the tendency to favor one leg when jumping, landing or pushing off of things, taking sharp turns etc.
On downhills: run more on the balls of your feet, slightly leaning forward. Running on your heels promotes a "braking" action which fatigues your muscles. Secondly, don't overstride. Let your feet touch down under you, not way out in front. Keep your arms slightly outward (like you have wings) and across the body for balance, and use gravity to your advantage.
On uphills: shorten your stride for lower cadence and keep your head up and shoulders back. Put your chest forward and use your arms in a straight forward and back pumping motion. Concentrate on good form and lifting your knees. Be positive and embrace the climb. Running uphill is easier on the body with lower impact. Short uphills are more about power and more arm action while longer uphills are more about being efficient and finding a rhythm."