Summarize this article:

When you're hiking, you'll appreciate having the lightest possible pack on your back. Choose the smallest and lightest backpack you can find that will hold all the supplies you need for your journey. If you're just going for a long day hike, you can get away with a smaller pack, but for an overnight backpacking trip you'll need a pack that will fit sleeping gear like a sleeping bag and tent, as well as plenty of extra food and water.  Backpack capacities are measured in liters, and you'll see backpacks for sale that can hold anywhere between 25 and 90. The average capacity for a day hike backpack is 25 to 40 liters (6.6 to 10.6 US gal), and the average for a hike that's five days or longer is 65 to 90.  Aside from the length of your trip, the other variable involved in choosing a backpack volume is the season in which you'll be hiking. You'll need a larger backpack for hiking during winter months, during which you'll need to carry heavier clothing and other extras. Most backpacks are manufactured with internal frames that help support the weight, though you can still find a few external-frame backpacks designed to bear the very heaviest loads In any case, rather than just carrying a standard school backpack, look for one especially made to carry weight while hiking for optimum comfort. When it comes to hiking you'll want to bring along only essential items. It might be tempting to bring along your camera, a journal, and your favorite pillow, but bringing unnecessary extras will weigh you down. Pack only as much as you need for the hike you're doing. Do research to find out what you should bring for the particular hike you're doing, taking into account how strenuous the hike will be, the number of nights you'll be sleeping out and the weather.  Consider springing for the lightest yet sturdiest gear possible, especially if you're going for a longer hike. For example, if you need to bring along a sleeping bag, you might want to get an extremely lightweight and compact bag weighing only a few pounds instead of bringing along a big, fluffy bag that will take up a lot of space and weigh you down. But, you should consider the weather, climate and terrain of where you'll be hiking. Sometimes, you may need bulkier items. Wherever possible, pare down. Instead of bringing along a box of granola bars, remove them from the box and carry them in a plastic bag. Instead of bringing a heavy camera, consider using your mobile device's camera. Some people even pare down by cutting off their toothbrush handles and snapping their combs in half. Spread out everything you're bringing and organize it into piles according to the weight of the items. Have a pile for heavy items, medium-weight items and small items. Organizing your items in this way will help you pack everything properly to ensure your hike will be as comfortable as possible.  Light items include your sleeping bag, light clothing, and other light nighttime supplies. Medium items include heavier clothing, your first aid kit and light food items. Heavy items include heavier food items, cooking supplies, water, your flashlight, and heavy gear. It's important to maximize space as much as possible and concentrate the weight. Consolidating items will prevent them from loosely traveling around your backpack. Your backpack will stay better organized and well-weighted if you take the time pack flexible into extra spaces.  For example, if you have a small cooking pot, fill it up before you pack it. Stuff it with food supplies, or store your extra pair of socks there. Maximize every little bit of space you can. Pack small items that you use at the same time in the same place. For example, pack your toiletries in one lightweight bag to keep them all together. This is a good opportunity to eliminate items that are taking up too much space. If you have an item that you can't easily pack in with the other items, because it's an awkward size or made of inflexible material, you might want to leave it behind.

Summary:
Choose a backpack. Gather necessary supplies. Lay out your supplies by weight. Consolidate items wherever possible.