Ditch the watch or at least your ego.
The nature of varied terrain—uphills, downhills, roots, rocks—means every mile will be different and very different from anything you run on the road.
Your body will better adapt to any potential misstep. Focus on keeping your shoulders loose and not hunched up by your ears.
Choose a line
Look a few paces in front of you, and focus on where you want to run (as opposed to that pile of roots you want to avoid). Your body will follow.
Widen your arm swing
This gives you more balance on the trail; it also helps in keeping your chest open, letting you breathe more freely.
Drive those arms
They’re like pistons: Power comes from your arms and transfers through your body to help propel your legs. Especially on long climbs, focus on a powerful arm swing.
When running up steep hills, avoid leaning over too far at the waist. Bending over too much compresses your lungs.
Keep your hips under you
On all surfaces but especially on downhills, aim to keep your hips in line with your shoulders instead of sticking your rear end out behind you, tucking your tailbone, or leaning too far back.
Shorten your stride
Take quick, short steps and land flat-footed. This enhances agility as you pick your way through trail obstacles—and can keep your quads and hamstrings from screaming during a long descent.