two tips for bicycle commuters...
1-carry everything you need
make sure you have a spare tube, patch kit, and a multi-tool
2-don't carry anything you don't need
leave your lock at your destination
leave your work shoes and other clothes at work instead of carrying everything back and forth!
I totally dig my ERGON pack
the design of this pack does a fantastic job of putting the weight on the hips while keeping space between the pack and the back for breath-ability!