Choose a good quality pack.
Many inexpensive $30-$50 packs look good, but won't hold up or be comfortable. The last thing you want is a broken pack to lug back from your hike. Trust me, it is worth the extra few dollars
This is an example of a cheap pack. The padding is soft like flannel. It looks nice but won't last. We have tried these packs, and they are not very good.
This is a good pack. Notice the strength of the material. It is a very durable nylon mesh. The packs we sell are made of high quality materials.
Notice the nylon mesh and the ripstop nylon, instead of soft flannel.
Internal-Frame Packs
Internals feature a narrow, tower like profile and integrate their framework inside the pack, behind the shoulder harness. The frame usually consists of "stays," or flat bars, about an inch wide and 1/8-inch thick,
Internals are popular packs with many advantages:
Flexibility. Stays make internals stiff, but not rigid. This allows the pack to move in harmony with body movements, a big plus for climbers and skiers.
Balance. Internals hug your body. This holds your equipment closer to your natural center of gravity and helps you keep your balance when it counts — for example, while you're scooting across a log above a stream.
Stability. Compression straps are everywhere on an internal. You use them to cinch down your load and keep individual items bunched together. This keeps them from shifting and throwing you off balance if you make any abrupt moves.
Maneuverability. Because internals feature a slimmer shape, it's easier to swing your arms freely — another reason why these packs are popular with climbers and Nordic skiers. This narrow profile also helps hikers whenever they have to squeeze through tight spots or when they're bushwhacking through thick brush.
Adjustability. Internals use suspension systems (involving the shoulder harness and hipbelt) that can be adjusted more precisely than external-frame systems.
Day use. 2000-3000 Cui.
Multiday. 2,400 to 4,200 Cui. load between 37 to 57 pounds. These are a great choice for 2- to 5-day outings.
Extended trips. 4,500 to 6,000 Cui. for loads of more than 50 pounds. They feature relatively stiff internal frames and plush suspensions. Ideal for backpacking trips of a week or longer, they are also a good choice for shorter spring and fall trips when you need to carry more clothing.
Loading a pack
Whether you're traveling on- or off-trail, keep your heaviest items close to your back, centered between your shoulder blades.
For on-trail travel, keep heavy items higher inside your pack. This helps focus more of the weight over your hips, the area of your body best equipped to carry a heavy load.
For off-trail exploration, reverse the strategy. Arrange heavier items lower in the main compartment, starting again from the spot between your shoulder blades. This lowers your center of gravity and increases your stability on uneven terrain.
Stuff your sleeping bag into its lower compartment first. Squeeze in any additional lightweight items you won't need until bedtime (pillowcase, sleeping shirt, but nothing aromatic). This will serve as the base of the main compartment, which you'll fill next.
Tighten all compression straps to limit any load-shifting. 
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