There are two goals when packing your backpack: 1) to distribute 80-90% of the weight on your hips, and 2) to make your gear logically accessible. Follow these tips to accomplish each goal.
PROPER WEIGHT DISTRIBTION
How you pack your backpack first depends on whether you have an internal or external framed pack. Generally, you want to keep the heavier items close to the center of your back.
For external framed backpacks, you want to place the heaviest items in the top third of your backpack, centered towards the back. Medium weight items should be placed in the middle of the pack. The lightest items, such as apparel or your sleeping bag should be placed in the bottom.
With an internal frame pack, this distribution changes slightly. Your heaviest items should be placed in the middle center of the pack no higher than your shoulder blades. Medium weight items should be packed around the heavy items, both above and below the heavier items. The lightweight items should be packed at the bottom of the backpack.
There are great illustrations on these weight distribution strategies at:
http://www.rei.com/online/store/LearnShareDetailA rticlesList?categoryId=Camping&url=rei/learn/camp/pa cking2f.jsp
WHEN AND WHERE YOU NEED IT
The second consideration when packing your backpack is to place items where you can access them when you need them. You don't want to be searching for your first aid kit when an emergency arises. Your first aid kit should go in an outside pocket or near a backpack opening. Usually, clothes go in the bottom of your pack, but you'll want to make sure your rain jacket, for example, is the last of your clothes to get packed away, so it's on top.
OTHER CONSIDERATIONS
You'll be packing items that can spill, such as fuel and food, and ruin other pack contents. Make sure these are packaged in spill proof packages like waterproof stuff sacks or Ziplock bags.
Stuff sacks come in a variety of sizes and colors. You can organize the contents of your backpack by using stuff sacks and choosing different colors for particular types of items. The stuff sack is also great for stuffing clothes in and using as a pillow.
Always pack a repair kit. A little rope, duct tape, and a few safety pins can go a long way. Here's a link for a sample repair kit:
http://store.everestgear.com/117581.html
Happy trails!