Here is a screenshot of my current setup: http://puu.sh/cOVPJ/ea4bc82f95.jpg
I set up my mouse in a way that I can switch the top left bar in the screenshot with my mousewheel:
- mousewheel down switches to bar 1 (1 - Heavy Swing, 2 - Skull Sunder, 3 - Butcher's Block, 4 - Fracture)
- mousewheel up switches to bar 2 (1 - Heavy Swing, 2 - Maim, 3 - Storm's Path, 4 - Storm's Eye)
- pressing the mousewheel switches to bar 3 (1- Inner Beast, 2 - Steel Cyclone)
Other than that the second row is bound with the alt modifier, the third row uses the ctrl modifier.
Edit: Found a better screenshot.