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.
They're one of the most difficult muscle groups to develop, but the cure to your underdeveloped hamstrings might lie in the answer to this question: "Which way do you point your toes during a leg curl?"
You've seen his name on a lot of recent T-Nation articles, and you've seen him pass out some outstanding training advice on the forum. And you've probably thought, "Man, that guy is smart, but who is he anyway?"