Hold heavy stuff and do plate pinches. (Turn two 10s against eachother and hold them from the outside)
Odd object training also helps significantly.
But seriously a deadlift will help your grip strength more than any wrist curl. I remember when hanging onto a couple hundred pounds was taxing on my grip. Lots of heavy lifting later.... 375lbs at lockout is not a problem in the least to hold onto.
Snatch grip deads will crush your grip like nothing else.