Hold one side in an isometric contraction while the other arm moves through a dynamic (concentric and eccentric) movement. You then switch sides.
This simple modification not only helps you to create a new training stimulus, it also creates muscular overload by increasing the time under tension (TUT). By increasing TUT, you increase metabolic stress, a key mechanisms for growth.
Dan John's been dragging his calloused butt across Terra Firma for a relatively long time, but he only recently realized he'd been wasting a lot of time on assistance exercises and aerobics.