I was going to say "masterbating helps', but that would be crass.
So -
What I used to call 'roll-up' - using light dumbells, place you arms over a bench/table, roll up wrists, forward and back. Good for forearms.
Sqeeze balls/grips for fingers/forearms.
Pushups also work your forearms and fingers, if you want them to. In fact, lots of standard excersises can incorporate a lower arm action, like flys, curls etc.