Don't ditch the messenger bag until you try it. I've gone on 20+ mile errand runs with my bag carrying all sorts of stuff. Unlike a backpack, most of your back can breath, so it's not that hot. If you have a bag like Timbuk2 it will have a second strap that connects to the first, which keeps the bag in place. Just experiment a little to find the right position for it. I've tried panniers, and I just can't get used to the way they make the bike handle.
Then again, I ride a single speed with 700x23C tires, no fenders or rack, and wear bike shorts and a jersey everyday. So I might not be the "typical" commuter (if there is such a thing).