Greet hikers you encounter. The A.T. is known as a friendly place; help keep the tradition!
Yield to uphill hikers. When you see someone coming, look for a rock or bare ground off the side of the trail so you don't damage moss or crush plants when you step aside.
Avoid cutting switchbacks. You might be eager to get back to your car and and kids might want to have a race down the mountain, but taking a shortcut on those hairpin turns in the trail leads to erosion.
Keep your dog on a leash, and pick up after your pup. Find more tips here.
Be prepared for bathroom breaks. Make sure you have a toilet kit with a toilet paper, a zip-lock bag, hand sanitzer, and a trowel to dig a hole if you have to poop. When you have to go, make sure you are 200 feet from water. Make sure you carry out all paper, wipes, and feminine products. Find more info on how to dispose of waste properly here.
Carry out your trash (you might want to bring an extra zip-lock or plastic grocery bag). You can be a "trail angel" by picking up any trash you see along the way or at the trailhead parking lot.