Start with adjustments. Loosen all the pack straps - including the shoulder straps and waist belt, fill the pack with about 25 pounds of gear, and put it on. For starters, cinch the hip belt so it fits snugly but comfortably around your hips (NOT your waist - if it's around your waist, the pack frame may be too short). Next, pull the shoulder straps on snug. The straps should angle up slightly from the harness, then curve over your shoulders at about the middle of your collarbone. The padded portion of the straps ought to end 2-3 inches below the armpit. If the straps want to pull the pack up off your hips, you may need to take it off and lengthen the torso adjustments; if the straps aren't carrying any weight, shorten the torso.
With the pack back on and the hip and shoulder straps snugged, it's time to bring the load lifters into play. These are the "auxiliary" straps that extend from the shoulder straps to the top of the pack, and play a key role: They allow you to adjust the amount of weight your hips or shoulders carry. Pull them tight. They should extend at a 45-degree angle to the top of the pack. For more weight on your shoulders, pull them tighter; for less, loosen them.
Finally, adjust the sternum strap - the strap that extends across the chest. This helps position the shoulder straps at a comfortable width on your shoulders, and helps stabilize the load on rough terrain. Move the strap up or down to the most comfortable point, then pull snugly. When actually hiking, don't be afraid to unclip the sternum strap - it's a helpful accessory, but can inhibit breathing when you're working hard.
Now that the pack is adjusted properly, it's time to load up. There are two basic principles to follow: Keep heavy things close to your back and fairly high, and keep handy things you're apt to need.
Here's how I usually pack: If my pack has a sleeping bag compartment, I begin by stuffing the bag (already in a stuff sack - don't pack it loose) in there. There's usually extra space, which I fill with spare socks, a paperback book, long underwear - whatever fits. Put some muscle into it: You want this "floor" for your load to be firm and evenly packed.
I then zip shut the sleeping back compartment and start loading from the top. First to go in: extra clothes that I don't expect to need on the first day's hike. Again, pack firmly - tightly roll your clothing first, then stuff it into the pack with some authority. Small stuff sacks can help you organize clothing - one for long underwear, another for socks. Next are the main food bag and cooking utensils such as a pot-and-pan set. Make sure the pot is oriented so that it lies flat against your back. Then, place heavy items in the pack and close to your back - things like a fuel bottle and stove. If you're going to be on rough terrain, place them at about the midpoint of the pack. If on easy terrain, higher up. Lastly, fill the top of the pack with things you're apt to need that first day: Rain parka, gloves, lunch, first aid kit, camera, headlamp. I usually put water bottles in side pockets. Into the top pocket go small accessory items such as knife, compass, map, emergency whistle, anti-blister supplies.
Remember to pack things firmly, particularly if you're using an internal-frame pack - the load actually becomes part of the frame, and will carry much better than a loosely packed pack.
Once you're packed, snug the compression straps that encircle the pack. These will help stabilize the load, and help bind it to the frame suspension so it carries better. Adjust the lid straps so that you have room to move your head around and look up. Put the pack on, re-tighten all the suspension straps, and check for balance and pointy things jabbing you in the back. Adjust as needed, and off you go!