Bear Bagging is a technique to make sure that your food does not end up a bear's late night snack. It also makes it nearly impossible for the other nighttime creatures to steal some of your grub. It is not just a recommendation but a requirement in some areas. If backpackers get lazy and bears figure out that backpackers = food we will be creating "problem" bears. Bears that become dependent on us for food are potentially dangerous to us and leads to bears losing their natural food foraging ability. It is important for backpackers to respect the environment they are in and leave it the way they found it. This includes making sure that our impact on the native wildlife is as close to zero as possible.
The bear bagging technique is not bear proof, so in some problem bear areas you are required to use bear canisters. Bear canisters can be bought (at REI) or rented from the ranger stations directly. As always talk to a local ranger before hitting the trail to find out requirements and current conditions. One of the downfalls of Bear bagging is you must have a tree that meets certain criteria (described in detail below). If you are camping in an area that is above tree line, or simply does not have many trees, than a canister is the only good way to protect your food from critters.
Bear bagging, the right way, takes 50 feet of parachute cord (thin but strong rope sold at most camping equipment stores, cheaply), a carabineer (not required but makes it easier), and a trekking pole (so you don't have to hunt for a long enough stick around camp). You also need either one or two stuff sacks big enough to put all of your food, trash, any containers that have or had food in them, toothpaste, and toothbrush. Bears are attracted by smell so anything that smells like food to them you want up in the tree.
Bears are very intelligent so if the critter you are concerned about happens to be a bear than the best way to bear bag is to use the counter balance method. Find a tree that is 200 feet or more from camp that has a live branch that is at least 15 feet up from anything a bear could stand on. This branch also needs to be strong enough to hang your food on, 10 feet from the tree trunk, and not strong enough to support a bear's weight. Usually a branch that is at least 1 inch at the rope point and 4 inches at the tree trunk will work. You also want to be sure there is not a branch around this one the bear could climb out onto and get your food. (the bear canister is looking easier now isn't it.)
Once you have located your tree hopefully while it is still light. I have bear bagged at night but it is much easier to have your rope set up and ready to go before you actually need it that night. Get your rope and attach it to a small rock using any effective knot you know. An easier method that a fellow backpacker emailed me is to use an old tube sock to put the rock into and then tie the rope around the sock. This is easier that having to tie a rope around a rock directly. Of course part of the fun with bear bagging is the rock throwing and the challenge that comes along with it.
You will be throwing this rock with the rope attached over the tree limb described above. Now this step is usually the one that is the most fun if you are willing to laugh at yourself. Throwing a rock over a tree limb seems like a simply enough task but do not expect to get it on the first try and make sure that the path is clear of all people in case your rock escapes your knot and just keeps on trekking. Once your rock is attached to your rope make sure that your rope is free from any knots and that you are not stepping on it. You want the rope to be free so when you throw your rock it can gently pull the rope up and over the branch. If your target tree limb is especially high you may want to step on the free end of your rope as you throw. Leo likes to have the free end of the rope in his non-throwing hand. Do whatever works for you.
When you are not happy with the position of your throw go ahead and pull on the rock end of the roper until the free end goes up and over the branch and then reset and try again. If you pull on the free end the rock could get tangled in the branch and then you have a big problem, Once you have made a throw and you are happy with it you should have something like this. Leo is gently pulling on both ends of the rope to check the branch for strength. Once you are satisfied that it will hold you can leave the throwing rock attached and then tie off the free end on a tree (even the same tree) until you are ready to hang your food. In our case we knew that instead of using two stuff sacks all of our stuff would fit in one so we got a rock that weighed about what our stuff would later (to counter balance it) and tied it to the free end. We then untied the throwing rock and tied that end to a tree. Tying the free end to a tree makes sure that your rope does not get tangled in the wind. We usually will tie a small loop and anchor the carabineer to the side we will attach the stuff sack later. The loop provides a spot to grab with your trekking pole to pull down the stuff sack the next morning. Do not forget to have a loop on one end of the rope for this reason.
When you are done with all of your food items and toothpaste you can hang your food. You will need to either evenly distribute it to two stuff sacks or put it all in one and get a rock of about the same weight. You need to attach your stuff sack or sacks to the rope using a carabineer (or knot). You want to attach one stuff sack (or rock if you are using the one stuff sack method) to one end of the rope. Usually this side of the rope will have a loop on it to allow you to pull down the food the next morning. Pull the other end of the rope until the stuff sack (or rock) is almost at the branch. Now you will need to make a loop in the rope you are pulling at the highest spot you can comfortable reach. Attach the carabineer to this loop with your other stuff sack (or your only one) and put any free rope into the stuff sack before closing it completely. You are ready to use your trekking pole to push the stuff sack up so that it is the same height as the other stuff sack. Now obviously you do not want to push the stuff sacks (or sack) any higher than you can reach with your trekking pole or you will never be able to retrieve it.
When you need to get your food down use your trekking pole to pull on the loop so you can reach the stuff sack. Untie the stuff sack so you can lower down your other stuff sack (or rock).