To add to Grouseman's comment......
Nick Waller is who I deal with at River Sports. He is an organizer with the Great Smokies Hiking & Adventure Group and knows his stuff pretty well. I think he is also a Gregory certified pack fitter, and has taken the Go-Lite efficient packing class. As far as backpacks go, River Sports carries Gregory, Arc'Teryx, Go Lite, Granit Gear, Osprey, Lowe Alpine, & Mountain Hardwear.
I would do some research on the internet, figure out what features you want, pick a few different packs out, and go to the store with the intent to load them and try them on. Don't go to the store with a clean slate....your opinions on what you need are equally as important as the sales associates.