2. They underpack or overstuff.

You don't want to end up with 25 pounds of equipment for an easy afternoon hike—and you don't want to be caught empty-handed in an emergency. For a day hike, Pohja's pack always contains these essentials:

• A map and a compass. Pohja says that national parks are especially good about keeping their maps up-to-date.
• Sunscreen and sunglasses, especially if you're hiking above the tree line. There's less shade, and UV rays are stronger at higher elevations.
• Fluids. She recommends 3 to 4 liters of water for each person in your group. "Many people think that they can fill up their bottles in a river, but drinking untreated water can put you at risk for waterborne diseases."
• Snacks. Instead of breaking at the summit for a big lunch, NOLS instructors like to nibble throughout the day. This way, Pohja says, they maintain their energy levels and avoid the discomfort of overeating. She likes snacks that combine carbs for energy as well as protein, like trail mix, jerky, and crackers and peanut butter.
• A first aid kit with Band-Aids, athletic tape and an antibiotic ointment to clean wounds.
• A rain jacket, regardless of the forecast. "Afternoon thunderstorms are common in the mountains, especially in the West, and a jacket is a small thing that will make a big difference."
• A headlamp or flashlight in case you get stuck outside after dark.

3. They think hiking poles are for sissies.

Pohja recently helped lead a group of Naval Academy students on a 30-day wilderness trip in Wyoming. "All of our instructors had poles, and we told the students they could bring them, as well." None of the midshipmen did, and after a few days of picking their way over boulders and down snowy slopes, they were all confessing to pole envy. Poles take some of the weight off your knees, hips and ankles when going downhill; they're highly recommended for hikers with knee or hip issues. Pohja says poles can also help redistribute your weight while walking uphill: "You're engaging more of your upper body and arms." You'll burn more calories this way, which is a benefit if you're going out for a fitness hike.

4. They can't read a map.

Pohja says she has run into strangers in the woods who try to make the map fit the terrain around them. "They think they know where they are," she says, "so they look at the map and say, 'This peak must be that one there; this pile of rocks must be that one over there...' It's easy to convince yourself and others in your group that you know exactly where you are." Your instinct may be to keep forging on optimistically, even if the markers on the map don't match up exactly to what you see in front of you. But be ready to change course. Pohja says, "If the map says you should be hitting a bridge soon, and if you don't hit it in the next 20 to 30 minutes, stop and think about where that bridge might actually be."