REI released a new infographic on choosing and using a backpack.
(Click here to see the full chart.)
This chart includes everything from choosing your pack size and finding the right fit, to packing it correctly and deciding what to leave at home.
It's a pretty handy tool, but I would add one key piece of advice:
Unless you're a veteran backpacker who knows what she wants, always try on your pack in a store, with the help of an expert, before buying. Packs—and bodies—come in all shapes and sizes, and a too-loose belt or too-long torso can lead to a lot of pain on the trail.
Use this chart as a guide, but use your local outdoor store to make the final purchase.