Pack the lightest equipment; sleeping bags, pillows etc, at the very bottom of the backpack. Where you pack the rest of the equipment depends on the kind of hiking you are doing. If you will be walking mainly over level ground, pack the heaviest items in the top third of the backpack as this will bring the weight over your hips, making it easier to carry. However, do not pack heavy items above shoulder height as this compromises your balance.
Pack the heaviest items lower down if you will be hiking over difficult terrain than you would if you were hiking over flat terrain. Place them in the back pack between the bottom half of shoulder blades and the top of the hips. Women have a slightly lower center of gravity than men, so they should pack heavy items slightly lower to maintain balance.
Pack the weight so that it lies as close to the back as possible and is aligned centrally. Lopsided backpacks will give you blisters and ruin your balance. Use lighter items to stuff between the heavy item and the wall of the backpack to keep the heavy item pressed in place.
Stand the backpack on the ground and hold the straps out in front of it. Note the highest point on the curve of each strap. There is often a plastic tab at this point to help you locate it. Place the backpack on your back and adjust the straps so that the top of the curve is positioned directly over the center of your shoulders.
Position the lumber support, if the backpack has one, so that it fits comfortably against the small curve of your back. Lean forward and pull the hip belt around your waist, secure the buckle and tighten evenly on each side so that the fits tightly around your hips and the belt is about 70% over the top point of your hip and 30% below.
Stand up straight and feel the weight distribution. You should have around 80% of the weight on your hips and the remaining 20% on your shoulder, but to some extent this is a matter of personal preference. If the back is fitted to your height correctly the straps should still place some load on your shoulders but you may need to tighten them or adjust them where the straps join to the body of the bag. Lower the straps slightly until even weight distribution is achieved.
