Work on your flexors and extensors. They are the muscles on the top and underside of your arm that dictate your gripping power.
Wrist Curl (Do this kneeling on the ground with your arms supported by a flat bench instead of your upper legs)
Reverse Wrist Curl (kneeling and supported by a bench instead of sitting and supporting with upper legs)
Wrist Roller (This one is a favorite of mine)
And
these three