Swinging Arms: straight ahead, not sideways
Because many of our daily activities tend toward placing our shoulders in an "internally rotated" position (think how you hold your silverware, read a book, type on the computer, drive your car, etc.) our arm swing, while walking, have a tendency to reflect that. This presents itself as the arms swinging across the body from side to side. The weight of the arms moving across the body causes more twist and torque of the torso and is not as efficient at counterbalancing the swing of the legs. So, while you walk, bring your attention to your arms swinging straight forward and back (on the sagittal plane) and see if you feel a difference!