PFT don't be dissin on Timbuk2!
I have a timbuk2 messenger bag and usually stuff it with quite a bit of crap. It's waterproof, doesn't get dirty, has cool adjustment gizmos and is very comfy.

You need a wide and stiff strap. If the strap is narrow, or it's flimsy and "bunches up," it will dig into your shoulder. Then, you need a fat strap pad. Without a strap pad, the edges of the strap will dig into your shoulder. And you need a second adjustment point. Messenger bags usually have the mini strap that goes under your other arm and ties the main strap to one corner of the bag.

Without these 3 things, you either get your circulation cut off by a crappy strap, or you put unnecessary work on your arm muscles to keep the bag from sliding off your back...

Also, if you paid less than $70 for your bag you should probably get a decent one...