Keep in touch: When trying a new trail, go with someone who knows it--and bring a map, compass, cell phone, and whistle. If you do go solo, give someone a map of your intended route. Go to nps.gov for maps of national park trails, or plot your route at www.usatf.org.
Think time, not distance: Tough terrain and hills can double the time you need to cover a mile. So consider how long you want to be out. "Experienced trail runners cover about six miles an hour," says Scott Jurek, seven-time winner of the Western States Endurance Run. "Less-fit runners should target four."
Fuel up: Bring food with you, even on short runs, in case you're in the woods longer than expected. "Energy bars and gels are good because they're easy to carry and digest," says Monique Ryan, author of Sports Nutrition for Endurance Athletes. The carbohydrates will help you run and concentrate. Also, stay hydrated with small, frequent sips from a water bottle or hydration pack.
Watch the weather: Especially at higher elevations, temperatures can change and storms can roll in quickly, so plan accordingly. Even in the summer, Jurek brings a winter hat and a Brooks L.S.D. jacket ($85, www.brooksrunning.com), which is water-resistant and weighs only 3.1 ounces. Use sunscreen, but be careful not to apply too much; it can clog pores and cause you to overheat.
Train your feet: Get used to running on easy paths, then move on to more gnarly trails. "Take short, quick steps so you can react," Jurek says, adding that your stride rate should be about 90 per minute. Road shoes work, but trail shoes have more traction and protect your feet from rocks and roots.