The straps on side/shoulder bags often leave a lot to be desired. They can lack for comfortable padding, length, or have their adjustment notches placed in just the right spot to chafe your shoulders. Try attaching a camera strap instead for greater comfort and flexibility.
The straps and discomfort of my shoulder bag is what caused me, after a few painful lessons, to upgrade to a backpack. Had I had an old SLR or DSLR, or found a strap a a thrift or camera store, my Targus might have seen a bit more action. Photo blog Photojojo details the simple process of adding an eyelet to your shoulder bag to accommodate a camera clip. If you've made a similar upgrade to your shoulder/laptop bag, brag about it in the comments.
Reuse Your Old Camera Strap to Make a DIY Shoulder Bag [Photojojo]