the wrist rolling thing is good. we used to have a little machine for that in the gym.

reverse curls also work to some extent.

Try theis, rest your forearms on a bench palm facing up and off the bench. Take a dumbell in your palm and flex the wrist, roll it toward you. Then do it palm down. (I've trained neck the same way, lie on a bench face down neck & head free, lift the head. Turn each way on the bench and hit all for sides)

Another one for forearms and grip, You'll need a bench just below mid thigh level and an olympic bar put about 40k on it and just hold it for 60 seconds, don't rest it on your legs, that's cheating do this twice, so 2 reps of 60 secs. If you find that easy add more weight.