when you do pull-ups (if you do them)
drape two workout towels over the top bar on the squat rack, twist them up and grab them (instead of the bar) when doing the pull-ups...
this will build massive grip strength, and your forearms WILL grow!
On biceps days, I like to alternate between reverse grip curls (outer head and brachiallis) and wrist curls with barbell off end of bench (inner head).
I don't think direct forearm work is necessary if you're doing heavy rows, weighted pull ups, rack deads, BB shrugs, etc. Just don't use straps for those movements.