Decide what you will use this backpack for. If you plan to go on day hikes, a pack ranging from 500 cubic inches up to 2,000 cubic inches is a good choice, depending on what you will be carrying. If you are going on weekend trip, one to three days, think about a pack ranging from 1,800 up to 3,500 cubic inches. Going on an expedition? Consider 5,000 cubic inches or above.
Do your research. Decide what you want in a pack. A hydration port? Side pockets? Top, side or bottom access points? Lightweight? Inexpensive? Once you have an understanding of what you'd like, check out some outdoor websites and compare. Websites like REI.com have a wonderful comparison feature which lets you click multiple items and compare everything from cubic inches to access points to weight.
Try on several packs. The outdoor experts in a store have knowledge of packs and know how to properly fit you. Pick a few packs out that you researched and liked and try them on to see how they feel. Ask for some weight to be put in and walk around, bend over, turn, move your arms, do all the little things you would do out on the trail.
Pick your pack. Sites like EMS and REI allow you to pick backpacks online and get them shipped to the store for free. If you are going to a specialty shop, remember that the service and support you receive while deciding on a pack will be the same service and support you receive on future questions, products or return/exchanges.
