Here's a quick trick for making your laptop shoulder bag more comfortable to carry: add an adjustable strap from another bag you have laying around to turn it into a backpack. There are two ways you can position the straps.
Photo remixed from originals by Morris Rosenthal
In the first laptop bag conversion option, make the second strap cross over the original strap as in the picture above. When you slip the bag on, the straps cross your back. You'll want to adjust the straps' lengths so the bag feels comfortable on your back while walking.
In the second option, hook each strap to one side of the bag, creating a loop on each side. Then sling one loop around each shoulder. This version might not work as well if you have narrow or sloping shoulders, but you can hold onto the straps on the front as you can with a traditional backpack.
For both options, have padding on the straps—particularly when carrying heavy, large-screen laptops—is recommended.
Build a Laptop Backpack for Travel [The Laptop Repair Book]