Leave-no-trace camping is an increasingly popular – and necessary – approach to travel in wilderness areas. As the term suggests, the goal is for the camper to have as little impact as possible on the location he or she is visiting. One of its mottos is "Take nothing but pictures. Leave nothing but footprints." Its simplest and most fundamental rule is: pack it in, pack it out, but it goes beyond that.
The principles of "Leave No Trace" were developed in response to concerns that the increasing number of human visitors to previously uninhabited (or lightly inhabited) wilderness areas would destroy the very characteristics that make them attractive, and irreparably harm native species. While each individual visitor might have an insignificant impact, the cumulative effect of thousands and even millions of visitors to a site over the years would be profound. Rather than removing humanity altogether from these environments, leave-no-trace camping seeks to minimize the changes we make just by visiting. It's also a courtesy to the people who'll be visiting the same place the next day or the next week, giving them a chance to experience it the same way you did. And pragmatically, the authorities who manage the place you want to visit may require that you follow leave-no-trace practices.
Many places where leave-no-trace camping is required feature established trails. Stick to them as much as possible. Yeah, the motto says it's OK to leave footprints, but keep them where they'll do the least harm. Especially if the trail is narrow, walk single-file instead of side by side. If you come to a muddy section in a trail, walk through it rather than stepping to the side and walking around; if your boots can't handle a little mud, you wore the wrong footwear. Don't take shortcuts across switchbacks on steep slopes; not only does this cut new trails into the terrain, they'll be especially prone to erosion. The reasons you follow animal created trails in nature are that those trails might lead you to good water source or might help you to avoid hidden dangers like hidden pit or snake pit or dangerous plants in that area. But most of the time, a good animal trail will cross your path for a few minutes at best.
When walking in an area without established trails, the approach is almost the opposite. Rather than trying to concentrate your impact (such as on a single trail) the idea is to disperse it as much as possible. Try to avoid following trails others have left, because that tends to make them larger and more damaging. Several people walking single-file etches your shared trail more deeply, but each member of your party can instead tread more lightly on his own path. For that matter, keep your group small; split up a troop of 10 to take more than one route to your destination. Walk on the most durable ground you can: rock instead of dirt, dead grasses instead of live plants, hard wet sand instead of loose dry sand, dry soil instead of muddy, etc. If it helps, treat it like a game: pretend someone's following you and you want to lose them.
In an area with established campsites, leave-no-trace is generally as simple as leaving the site in the same condition you found it. Don't build a fire pit, don't haul a log in to sit on, etc. The Boy Scouts used to teach kids to dig a drainage ditch around their tent, but this is both hard on the site and unnecessary if your tent has adequate weather-proofing. (It's also no longer in the Boy Scout manual; they teach leave-no-trace principles now.)
If there are no established sites, take care in selecting where to camp. In addition to the pragmatic considerations of level ground with protection from the elements, again look for durability. Firm, dry ground with minimal vegetation is best. Avoid pitching your tent on fragile plants that won't be able to recover from it. A pre-existing layer of pine needles on the ground makes for a surprisingly comfortable mat. If you find a good site that looks like it's been used recently, don't use it; find another site and let this one recover a little longer. Stay some distance from lakeshores, where the ground tends to be more wet and fragile, and where wildlife might be used to visiting at night.
Consider eating only cold food. This has a lower impact on the environment and means you don't need to carry stoves and pans. Cured meats, such as biltong, are lightweight, high-energy, long-lasting and easy to eat on the go. Complement that with muesli, granola or trail mix and you almost have a balanced trekking diet. Sweets and dried fruits should complete your provisions.
For up to the first week or so, you can take fresh fruit, bread, and other semi-perishables. Oranges travel well, as do apples, soft fruits do not. Pack out any parts you can't eat (e.g. orange peels, apple cores). Although they're biodegradable, these shouldn't be left behind; if you leave them out in the open, they'll be an eyesore, and if you bury them, something's probably just going to dig them up. And you definitely don't want to risk seeds germinating and introducing a new species to the area.
Keep your food (and trash) away from the locals; the appropriate methods depend on what kind of beasties you're dealing with. Not only does this have obvious benefits for you, personally, but in addition, you don't want to encourage wild animals to associate humans with food. It's bad enough that bears in Yellowstone have graduated from stealing picnic baskets to breaking into cars; we don't want to train the critters of the world to follow people around looking for these amazing hot dogs they had once.
For longer trips, freeze-dried foods are the best option, being easy to carry, easy to prepare, and easy to keep from impacting the environment. Some are also pleasant to taste! Most commercial backpacking foods can be reconstituted with boiling water in their own packaging, which can be sealed up in a plastic bag after eating, for packing out. (You can reduce the packaging even more by transferring the contents to a sturdy – but less bulky – plastic baggie. As long as the baggie stays sealed, it'll easily keep for several days.)
Building fires is generally taboo, not just because of the risk of turning the place into a smouldering ash-heap, but because it requires collecting firewood and leaves half-burnt wood. Far better to use a gas-powered camp stove, which can weigh less than a pound and is generally more efficient at boiling water than an open fire anyway. If you have to build a fire, keep it in an existing fire ring or improvise a temporary one, use only small dead branches found lying on the ground, let them burn entirely, and scatter the cool ashes when it's done.
Washing up after dinner can be a little tricky. Ironically, using disposable "dishes" (such as the pouch your freeze-dried beef stew came in) is easiest on the environment in this situation, because all you need to do is pack them out (for landfilling at home). Coffee cups can be rinsed out with a little clean water and reused later by the same person without hygiene concerns. If you need to wash a cooking pan, first clean out as much of the food as possible. (You're hungry, right? Lick it.) Wash it with as little biodegradable soap as you can get away with, instead of using hot water to do the work. Scatter this dishwater away from camp sites and far from water sources. Better yet, bury it, which helps that soap to actually biodegrade.
Another washing method that does not involve soap at all is using sand or dirt. Fill your pan or dish with dry sand or dirt, and all oily remains will be absorbed by it. Rub off the dirt using either some small rock, a fork or your fingers. Repeat until you can no longer feel any residue on the surface, then wash with a little water to remove all remains of the dirt itself. It's even simpler when cleaning sealable containers such as cups with lids, reusable lidded containers and the like; just put some dirt in it along wish very small rocks and pour some water inside, then shake hard. The dirt will absorb anything oily, the rocks rub off the rough remains and the water ensures that all parts of the container are cleaned. This method is very effective for removing any remains from your dishes, but naturally it does not take care of germs. Therefore it is only effective when used immediately after the meal, before any germs could settle on the remains.
Brushing your teeth with your favorite minty-flavored, fluoride-enhanced toothpaste is a good habit. But you're not taking any serious risks of tooth decay or gum disease by brushing without it for several days, and that means one less strongly-scented germicide you'll be spitting into the environment. And if you floss, remember to pack the used pieces; you definitely don't want that stuff to become part of the eco-system you're visiting.
Water itself isn't a leave-no-trace issue, but if you're camping this way, you'll probably be drawing it from untreated sources such as lakes and streams. Check with local authorities to see what precautions they recommend for known pathogens in the area. Boiling it for several minutes, pumping it through a micropore filter, and/or treating it with microbe-killing chemicals such as iodine is usually necessary to make sure you don't catch something really nasty.
Stick with instant coffee and you won't have to worry about disposing of the grounds. Tea bags? Pack 'em out. Tang™, Kool-Aid™, and other powdered drinks are good if you want something sweet to cover up the taste of iodine. Needless to say, if you bring packaged drinks such as juice, soda/pop, or alcohol, take your empties with you.
Purists say that "pack it in, pack it out" applies to human wastes as well. After all, we do that for Rover the dog when we take him for a walk in the city. But for obvious reasons, most leave-no-trace campers aren't quite so meticulous unless it's required by the specific environment. Still, this doesn't mean you should relieve yourself wherever and however the spirit (or your bladder/bowel) moves you. Your wastes are not only distracting (especially to wildlife), but also a potential source of disease. Fortunately they're biodegradable and nature has systems for rendering them harmless over time.
Urinate at least a few dozen feet (~10 m) from any trails, and well away from water sources used by wildlife or fellow campers. Try to avoid urinating directly on plants, and preferably do it in a spot where it will either quickly dry on the rocks, or be soaked into the soil. Guys: spread it around.
Faeces should be buried, at least 200 feet (60 m) from water sources. Bring a garden trowel with you, so you can dig a "cat hole" to bury it in, at least six inches (15 cm) deep and wide enough that you won't fill it up. Fill the hole back up and "disguise" it when you're done. Don't try to pretend you're sitting on a toilet; squat all the way down over the hole. This minimizes the need for wiping afterward, because it's how humans were designed/evolved to dump solid wastes. Leaving your pants around both ankles can leave them in harm's way: the trick is to hitch them round your knees. Use (unscented) toilet paper as sparingly as you can, and either pack it out in seal-able baggies or bury it along with the faeces; burning it is risky.
Opinions vary on whether used tampons and sanitary pads can be buried (most say no), but non-biodegradable tampon applicators should definitely not be left in the wilderness. Women may choose to make use of reusable menstrual products: washable cloth pads will need to be cleaned with soap, but reusable menstrual cups made of rubber or silicon can be rinsed or wiped and cleaned more thoroughly periodically. Blood from the cups should be disposed of in much the same way as faeces.
If you want souvenirs of your excursion, either seek out a shop associated with the park where you're camping and buy your Death Valley snow globe there, or bring a camera and take photos. Maybe bring a little notepad and keep a journal of your adventure. Removing interesting rocks, artifacts, or other items means they won't be there for the next person to "discover". It might also be a violation of park rules, or even the law.