First of all, the type of backpack you are using determines how you will pack. An external frame pack distributes the weight differently than an internal frame pack. Youll want the bulk of the weight low with the external pack, since this helps your balance as youre hiking. With internal frame packs, youll want the heaviest items placed in the middle of your back close to your body. This method of packing keeps the weight close to your center of gravity and allows you to turn around without the backpack pulling you off-balance.
Usually, the two bulkiest items youll pack are a tent and sleeping bag. Sleeping bags designed to be lightweight for backpacking come with a stuff sack with straps to cinch down the size. You can use these straps to tie them to the frame on the external frame pack. For internal frame packs you can pack them in the bottom quite easily. If youre packing for a solo trip the bivy sack (all in one sleeping bag and tent) is very practical. If there are others in your party, its a good idea to distribute the different tent parts (tent body, poles and rain fly) among three people to spread the weight. You can stuff the tent body and rain fly into the top of a pack and tie the poles to the outside of a pack. Another reason to have the tent at the top of your pack is for easy access. Most people set-up their tent when they first get into camp. With the tent on top, youre all set and will not need to unload your pack to get at it. Also, if its raining, youll be able to get out of the rain that much sooner.
Ive found its best to roll your clothes and put the bulkier items (pants, shirts, sweat shirts, jackets) in a stuff sack. Small items like underwear and socks fit into zip lock bags and fit well in the outside pickets. Its always a good idea to have your socks and foot powder accessible when youre hiking. Also, dont forget to have a folding shovel and toilet paper handy. A zip lock bag to store the TP is very important.
Other items to pack are water, raingear, and food. A hydration system is crucial while youre hiking. Most packs have these configured inside the pack, if not; a camelback system will work underneath the backpack. A poncho that fits over your backpack is a good idea to consider. This can be stored in an outside pocket. If you plan on a sit-down lunch during your hike, you can pack this pretty close to the top of your pack. Energy bars work well, too. These take up much less space. Other food items and cooking gear are personal choice depending on how long youll be camping and what you prefer eating.
These are just a few tips to think about when youre packing your backpack. Youll come up with your own list before long and youll have a routine that fits you.