Litter
It should go without saying, but I guess it has to be said, because I still see litter along the trails: pack out what you pack in. Then add a double handful or two of other people's trash. On popular trails (popular with litterers, at any rate), I sometimes hike with a small plastic bag in my hands to accommodate the trash I pick up. That keeps my pockets from bulging with stinky cigarette butts and allows me to refrain from imposing on my companions by stuffing trash in one of their outside pack pockets. Burying trash is not acceptable: in most cases, it will soon be unearthed by rodents, birds or frost action and scattered in the four directions.
Even trash that remains buried can still come back to haunt you. By one estimate, a steel ("tin") can takes 20 to 40 years to decompose in a wet climate, 100 years to vanish in a dry one. A thin polyethylene bag will mar the landscape for 10 to 20 years; thicker plastics can easily take 50 to 80 years. Aluminum cans will scream "A slob was here!" for 80 to 100 years, perhaps as much as 500. A glass bottle may last 1,000 years – some people say 1,000,000, or just a few months shy of the Second Coming. While hiking in Rocky Mountain National Park one day, I spotted a bit of foil emerging from ground that was gradually eroding under the drumbeat of many feet. A little scuffing with my boot revealed an entire trash cache, including an empty bottle of Duffy’s Delicious Drinks. The trash must have been buried more than 20 years ago, but it was still perfectly intact.
Food scraps, including things like apple cores and orange peels, should also be packed out. Orange peels take anywhere from one week to six months to decompose beyond recognition. Don't feed the animals, either deliberately or inadvertently. (Actually, there are three exceptions, but I don't imagine you'll want to take me up on this. You can, to your heart's content, feed the mosquitoes, ticks and deer flies.) Feeding creatures that don't normally consume human flesh and blood disrupts their natural eating habits, makes them dependent on human food and turns them into pests that will eat holes in your pack in search of goodies. Bears that learn to associate people and gear with food can become so aggressive that rangers feel compelled to destroy them. When you get out of the woods, recycle what you can – your trash won't decompose any faster in the landfill than it will in the backcountry. Recycling saves energy as well as reducing the need for new landfills. For example, making aluminum cans from recycled aluminum uses 90% less energy than making them from scratch. The energy saved from making one aluminum can from recycled aluminum will operate a TV set for three hours.