Most hikers and campers have experienced the frustration of stopping on the trail and needing the one item that is buried at the bottom of your pack. Although you can't have everything at your fingertips all the time, a little bit of planning can go a long way when you hit the trail.
When packing your pack the first thing you should put in it is the last thing you will take out, typically that is your sleeping bag. To save room put your sleeping bag in a compression sack. If you're leaving for an extended period of time or are going into damp conditions consider getting a waterproof compression sack. If your budget doesn't allow it, a heavy-duty trash bag can also do the trick. Most backpacks with the exception of daypacks come with a separate section to store your sleeping bag on the inside of your pack and deep within. If you have extra room in this compartment, store your change of clothes and/or bed clothes. Just make sure that your rain gear, a warmer layer of clothes and an extra pair of socks are easily accessed from somewhere else in the pack.
The next bit of gear to stow is your tent or bivy sack. Some packs have a vertical compartment that you can store a tent in. If yours is equipped that way then you have the ideal place. If not you have a couple of choices. Some packs have straps where you can attach your tent on the outside. You also may want to consider stuffing your tent in a compartment in your pack and strapping the poles to the outside. Make sure your poles are in a stuff sack and well secured. Those pesky tent stakes that always seem to disappear should be placed in a sack with your tent. Consider carrying a couple extra stakes with you.
Next is your bedroll. If your bedroll is closed cell or open cell foam, simply rolling it up and stowing it on one of the outside straps is probably your best bet. If your bedroll is inflatable, be sure to store it in a tough stuff sack to keep branches or rocky edges from puncturing your pad, and leaving you sleeping on the hard ground at 2:00 in the morning.
Next is your cooking gear. If you don't plan to have hot meals while on the trail, consider storing your cooking gear and stove in one of the larger outside pockets your pack may have, but one that is out of reach. A word of caution about fuel, make sure your fuel supply is well stowed and away from your tent, bedding, and most important your food supply, especially if the supply is liquid like white gas or alcohol. White gas leaking can ruin the material of a tent, spoil food stores, and take the loft out of a sleeping bag. If your hiking in very cold conditions and your using compressed gas canisters, consider rolling one in your sleeping bag to help keep it warm during the day.
Next is your food. You should store it in two separate sections. For meals you may be preparing when you setup camp, a more out of the way flap or sack is an ideal location. For the snacks you will be eating on the trail, and that lunch you want to make on the run, keep it in a pocket you can reach around in, or even better a bag of gorp in the coat pocket for a pick-me-up without stopping. If your hiking in bear country consider storing your food in your bear sack. It will be a lot less work if you don't have to transfer your food stuff every time you set up camp.
If your pack is equipped with a removable daypack or fanny pack, store your essentials for survival, knife, flashlight, waterproof matches, survival candle, compass, emergency blanket, water purification tablets, survival whistle, first aid kit, and emergency rain gear in it. This way if you want to setup base camp and take a side hike in the afternoon, you can grab your daypack, some food, water, and map, and be ready to hit the trail.
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