One method would be working with clay. But if you'd rather avoid the mess, try this instead:
Place the fingertips on any handy wallspace, and keeping them nice & strong. you can step back and lean the upper body toward the wall, as though you are doing pushups on your fingertips. Small therapy balls or other "squishable types of items can help with grip strength as well.
But if you start getting any pain in the hands or forearms, pay extra attention not to injure yourself! There are some good books out on injury prevention & building grip strength which you can find by searching the subject on Amazon. Here are some good references for injury prevention & treatment: RSI Repetitive Strain Injury by Wendy Chalmers Mill. The Acupressure Warmup by Marc Coseo, or Acupressure's Potent Points by Michael Reed Gach
I strongly advise anyone with forearm & hand pain to work on themselves with their forearms: So if the pain is in the right arm & hand, use the left forearm & elbow to work all along the soft tissue from the wrist all the way up to the elbow on both sides of the right arm. It really helps break up the tension in the arms. Aside from that I also have clients, and other practitioners take used tennis balls and use them along the spine and other points on the body to open up tension in any area that needs it. The Acupressure Warmup is great for showing exactly what to do with this technique.