There's no one right answer to this question: The answer depends
on what kind of pack you have and where you're going to hike.
General principle: lighter-weight items go at the bottom; heavier
items at the top and close to your body. However, if you are going
to be scrambling or hiking off-trail on rough terrain or snow,
you might want to pack some of the heavier items a little lower
to bring down your center of gravity.
Sleeping bag: Most packs are designed with a
compartment at the bottom for your sleeping bag. If your pack
has dividers that separate the sleeping bag from the rest of
your gear, use them: It'll take some of the weight off your
sleeping bag. (Overly compressing a down sleeping bag can make
it lose its loft because it damages the down.)
Clothes go in next. But it's a good idea to stash a
few of your warm clothes¡Xhat, gloves, and an insulating
layer¡Xand your rain gear in an outside pocket where you can
easily get them.
Tents and tarps. There are a couple of choices for
your tent or tarp. Most packs have a separate compartment (where
the sleeping bags go) that is accessible via a zipper: If your
pack has such a compartment, see if you can stuff the tent under
your sleeping bag. That way, you don't have to unpack everything
to get your tent out if it's raining when you want to make camp.
This placement also works for packing a wet tent because it
won't drip over everything (Be sure, however, that the tent and
sleeping pack are in waterproof stuffsacks). Tent poles go
strapped to the outside: Tie them on tight!
Food, fuel, and cooking utensils. Try to store the
fuel upright and away from your food, in case of an accidental
spill. Many hikers put their bottles in an outside compartment.
These items can have hard edges: Be sure they're not poking at
Personal items. Squeeze these in wherever they fit.
Outside compartments. Use these for items you'll need
during the day: Rain gear, a few warm clothes, pack-cover, water
filter, water, lunch, snacks, TP, sunscreen, mosquito goop, and
Mattresses. Most hikers roll up their sleeping pads
and strap them to the outside of the packs. Some smaller
mattresses fit inside a backpack, where they are better
protected from accidental encounters with cactus needles and
barbed wire fences.