Travel with a partner.  The worst situation you can get yourself into is one where you run into an emergency and you're by your lonesome.  Before hiking out down the trail, give a copy of your itinerary to a responsible individual.  Think about including information such as the make, year, and license plate of your vehicle, the date you're beginning your hike, and the date you plan on returning. 

Assess your physical abilities.  Being in good physical condition is one thing, attempting a hike outside of your capabilities is another.  Group trips should always be planned out with the weakest member of the group in mind.  If you plan on doing a "big day hike" or a multiple-day backpacking trip, try to train for these adventures well in advance. 

Always think about your footing.  This holds especially true next to cliffs and on steep pitches.  By paying attention to wear you are stepping down, you can avoid rolling an ankle or worse.

Wear appropriate clothing.  Before hitting the trial, assess the season and potential weather conditions.  We always leave for a hike knowing that we packed too much clothing.

Check your gear.  Maintain your equipment in good working order.  You should always setup your tent, test your stove, and restock your first-aid kit before embarking on an overnight backpacking trip.

Look at the forecast.  Before trodding down the trail, try to catch a glimpse of the weather forecast.  If you are on an extended backpacking trip, take regular barometric readings off of your altimeter.  Look for high-level cirrus clouds, cloud rings around the sun and moon, and other signs of imminent bad weather.

Learn first aid.  A friend might thank you someday. 

Purify your water.  It doesn't matter how clear the water in the stream looks, it probably contains water-borne parasites or other harmful microorganisms.  We've seen hikers in dire straits because they decided that it wasn't worth the time to boil, purify, or tab their water.  Don't be a victim of beaver fever, treat all of your water.

Limit alcohol consumption.  Think before drinking too much alcohol.  Wilderness travel involves an inherent amount of risk - even before you throw alcohol into the mix.  If you do decide to bring a little bit of grandpa's old cough medicine along (as even we like to do), remember to keep everything within moderation.  Moreover, please pack out all of your bottles, cans, etc.