Just fill the pack full of books and walk around town for a couple hours. If something hurts, try a different pack. Rights Reserved A backpack is the only way to go as far as I'm concerned. Good packs are made by MEI, Eagle Creek, Jandd, Pangaea, and a handful of other companies. In the suitcase mode, I look a bit more upscale in hotel lobbies, and there are no important straps to tempt hungry baggage-handling machinery.
Some people take large stuff-sacks to cover their packs for flying.
Others just take regular backpacks and hope for the best. I pack heavier for camping and cold. To camp or not to camp, that is a question. Even if a friendly refuge could host me at the end of the trail, I sometimes prefer a lonely camp below a windswept ridge. These advantages clinch it for me.