As far as tips for picking out a backpack, the best one I can think of is to get the rest of your gear BEFORE you pick out a pack. Then, when you're ready to buy a pack, call the local outfitter, find out when they're least busy with customers, and take all your gear and food for a weekend, plus a liter of water, into the store and start trying on packs. The right pack is the one that fits. You want a pack that is big enough to hold everything inside; don't hang a bunch of stuff on the outside of your pack (especially don't lash a tent, sleeping bag, or sleeping pad to the bottom or back of your pack, unless it's an external frame designed to do that.) Stuff hanging outside will throw the center of gravity way off from what the pack's suspension is designed for, and it will never carry properly.

You also want a sturdy enough suspension that the pack won't collapse under the weight you put in it. A properly fitted pack should let you shift the weight from your shoulders to your hips, and back to your shoulders, just by tweaking the shoulder straps and load lifter tabs. A well-fitted pack will also be comfortable: it won't rub, chafe, bind, or slip downward on your hips.

It's kind of like being a princess in a fairy tale: you've got to kiss a lot of toads before you find the prince.