If you don't like using a backpack, I'm not sure a messenger bag is a good option. I use one for a 6 mile RT commute, but even on that short ride it's uncomfortable if heavily loaded, and sweaty in hot weather. I use it 'cause it's more convenient than panniers off the bike, but in general I'm in favor of letting my bike carry the load, and not my back.

A cross strap is essential for keeping a messinger bag from shifting while riding. The Timbuk2s and most other brands have this feature.