Not to be obvious, but... Run more trails. :-)
I'm a fairly skilled trail runner, especially given how relatively poor my road results are. My main pointers are:
1) Hills. Walk them. There's always this testosterone-fueled HR-pegging pace up the hills in shorter trail runs, and it's dumb. Wear a HR monitor and time yourself on a steep (20%) hill. You will have a lower HR and easier breathing at the SAME PACE if you hike it. Don't give up and walk easy; learn how to power-hike smoothly with loooong strides.
2) Watch the trail. It takes a lot of practice, but you need to be able to keep your eyes up on the trail well ahead of you, and not stare at your feet. Your body will process the data and put your feet in the right places. This goes double on...
3) Descents. Blast them. Learn to let the legs spin and freewheel down safe stuff, and use full-contact descending on rougher stuff. You need really fast feet, moving from good footing to good footing, and hands that aren't afraid to grab trees or roots.
Like anything, these skills are learned over time. Spend as much time as possible hiking and jogging easy on real rough terrain, while focusing on being smooth and efficient. Picture yourself flowing like water over the trail. Too many people react to hard terrain with explosive bounds and slamming feet - that will just kill your legs and expend energy. Slow is smooth, and smooth is fast.
Finally, buy super-sticky trail shoes. LaSportiva rules - I suggest the Bushido or Mutant. Having total trust in your foot placements is imperative!
Also, where are you located? This is good general info, but centered on my northeast experience. Front Range or Florida trail running brings different challenges.