I recommend a timbuk2 bag. The large size messenger bag fits about 2 full outfits, shoes, and a sweater/jacket. I've had mine for 10 years, and it's gone on over 120 flights with me. Not much sign of wear - the velcro still grips, and the clips are going strong. In fact, I just looked and timbuk2 is now making actual luggage. Though, the advantage of the messenger is the lack of internal structure. I've always found a space in the overhead bids, even when roller bags are being turned away.
If you're looking for a actual backpack, with 2 straps, go with Sandpiper of California (SOC) bags. Them things will never die. And you can velcro your name onto them. win-win!