First of all, is getting it on, of course. A lot of times they're fairly heavy, and you don't want to injure yourself in the process, so take a wide stance. Bend your knees a little bit, keep your back straight. Lift it up onto one shoulder, then shoot your arm through the other shoulder strap. At this point, what you want to do, is shrug your shoulders, so that you can get your waist belt hooked up, and then tighten it up, to where most of the weight is on your hips. At that point, you can go ahead and cinch down the shoulder straps, to pull it to where you're just the slightest bit of load, on your shoulders.
Then, you take these side cinch straps here, they pull the load into you, to get it closer to your center of gravity, until it feels comfortable, and close to you, where you can move around, and not feel like you've got an awkward load on you. It's a good idea to connect this breast strap across here. It keeps the shoulder pads from creeping out, and really helps support the load quite a bit, and keep it centered on you. Once you've done that, you also want to tighten up these cinch straps on your waist belt, so that the waist belt will fit you appropriately, and help carry the load, and you want to have, when you're done, most of the load on your hips with your shoulders picking up just a little bit of the load, and keeping the load close to you. You should have freedom of movement, and be able to walk comfortably, and not have to lean too far into the pack, and that's how you wear a backpack.
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