While not specifically grip strength, I think it helps with general wrist and hand strength: get a small (5 pound or so) hand weight and sit leaning forward with your elbows and part of your forearm on your legs. Hold your hand palm down with the hand weight in it and then drop your wrist down. Bring it back up as high as you can without lifting your arm, and then down again. Then, turn your hand over and do the same thing, curling your wrist up as high as you can with the weight in it. I also sometimes turn my arm on its side and and do the same kind of motion (which in this orientation is sort of like the wrist-only part of ringing a hand bell). I've been finding that these are increasing my wrist, forearm, and hand strength a fair amount over the last few months.