Keep it Clean
Carry in, carry out. This is an obvious one but it is easy to overlook or make excuses. Don't leave garbage, food, or food containers on the trail or anywhere in the woods. Also, don't leave your pet feces behind. Clean it up and dispose of it properly.
Stay on the Trail
There is no trail obstacle that should cause you to walk off the trail. If the trail is too muddy or wet, perhaps you should turn around. If it's just a small area of mud, walk through it. Walking off trail or creating shortcuts widens the trail and can be harmful to the surrounding plant life.
Don't Take Anything
Don't take anything from the natural environment, unless you're cleaning up what doesn't belong. As pretty as those exotic flower are, they belong right where they are, flourishing for others to enjoy.
Read and adhere to any trail rules and regulations. Though they may or may not impede your plans, trail rules are there for good reason. If the trail is closed, it may be too muddy or snow-covered to hike. Following the rules will benefit the trail as well as you.
Bury any waste at least 6 inches deep. Be sure that it is 200 feet or more away from any water source, trail or campground. Carry out toilet paper and other hygiene products.
If you're using a vehicle, practice safe, low-impact techniques. Do not skid out on turns and stay in the middle of the trail without veering off. Maintain control at all times. Yield to hikers and equestrians. If a trail is too muddy, avoid riding it until it dries.
Always respect other trail users. Low impact isn't just on the physical trail, but also the overall environment. Don't yell and make a lot of noise. Keep respectful of the peace and tranquility that nature offers.