Sam and Ginger tried to drill this into me, but 165 blisters later it finally sunk in.  Keep your hands light on the grips.  Your fingers should act as nothing but hooks on the handles.  If your forearms are wearing out early in the row AND you’re getting blisters every time you row you are gripping way too hard. 

Try this - from a dead stop drive with nothing but your legs.  Hang onto the oars without a death grip on the handles.  Just curl the ends of your fingers over the handles and let that be the only connection between your legs and the oars.  No forearms, no biceps, just your arms extended as if you were hanging on to the oars like a pull up bar.  Relax your forearms and your wrists and just let your hooked fingers transmit the drive of your legs to the oars.

It takes a bit of focus to come to realization of the concept.  The best exercise I’ve found is to repeat the exercise from a dead stop.  It will reinforce the negative aspects of introducing your arms too early in the drive, and give you a better feel of how little is actually required of your hands to “hang” on to the oars during the drive.  If you find yourself getting out of control when driving from a dead stop it is almost a guarantee you are either trying to drive with your arms too early, death gripping the oars, or a combination of the two.  Just relax and enjoy the ride.