Ok, so it's pretty amazing how many people
I've met in the last few months who don't have a clue as to how to pack a backpack for a multi-day trip. So, I figured I'd node a little HOWTO.
The first, and most important thing, is to make sure you won't be overloaded. Most people can comfortably carry the lesser of 60-70 lbs or 40 percent of their body weight for a few days - although this varies considerably with your physical condition. Basically, you shouldn't have to strain to lift the pack or put it on. Remember to budget weight for water - you should be carrying at least 2 liters at all times, more if you're in a water-poor environment. (see eating icicles to avoid dehydration for an explanation of why knowing where your water sources are is important)
Most of the weight will be resting on your hip straps, so the center of mass of the pack should be as close to the hip straps as possible. This means that heavy gear, such as cooking equipment, should be low in the pack. The sleeping bag usually goes at the bottom, in a stuff sack. If you don't have external pouches for water bottles, this is also the place to put your water.
Next, put your stuff in, with the most frequently used items on top. This saves you unpacking your pack all the way to get an extra layer of clothing if you're chilly. Make sure to compress everything as much as you can - pack space is valuable!
The last thing to do is make sure that a) your emergency gear and flashlight, etc. are at the very top and b) your pack is balanced left-to-right. You don't want to be leaning to one side at the end of the day.
Once you're packed, put the pack on, tighten your hip strap first, then adjust your shoulder straps so your pack isn't bouncing against your shoulders (it shouldn't be tight, but you shouldn't be able to get more than a few fingers in there). Finally, tighten your sternum strap and you're ready to go!