another good device for hand-grip strength (and your arm/shoulder for that matter) is the
Dynaflex "Powerball".
Some folks just can't get 'em working but if you practice you will find it's a grip/arm workout that will just wear you out! Great for improving grip strength and endurance.
It's amazing that such a small device will work you so hard but it does.
Additional info here:
http://en.wikipedia.org/wiki/Powerball_(exercise_tool)