Also work on squeezing your fingers together like theirs are not there. the squeeze causes a compression and then you can work the rotation for the win .
I like the squeeze as it addresses tendon strength in a direction not normally addressed.
Also as mentioned Cane work. Strike a heavier cane into a heavy bag will help build grip strength.
It will also train you to relax your grip and tighten for the strike. A constant tight grip can cause cramps in the forearms.