If you lift weights, a simple way to incorporate grip work is to use a thick bar or to wrap something around the bar to increase the diameter, such as a towel. There are also
grip wraps / sleeves designed for this purpose that snap onto the bar. Shrugs will work your grip directly.
Plate pinches ( like this -
http://gripsite.com/?p=93 ) work the muscles that are used to rack a slide.
For someone with a weak grip, or a recuperating hand, there's a product known as
Theraputty that comes in various grades offering greater or lesser resistance to deformation. You can use this for squeezing and pulling exercises.
Another option is a quality hand gripper like those offered by CoC -
http://www.ironmind.com/ironmind/ope...nsofcrush.html or the
Ivanko Super Gripper.
And remember, forearm and wrist strength contribute to grip strength - it's not just your hand muscles. So any grip training should incorporate these muscles as well.
Lastly be careful and go slow. The muscles and tendons that contribute to your grip don't seem to give you the feedback that larger muscles do, so it's easy to overwork your hands and hurt yourself. I've done this myself and didn't see it coming because my hands didn't feel overworked.