Depending on your predominant forearm muscle fibre type (fast or slow twitch) you might benefit from higher repetition work every day (or at least 5 or 6 days a week). I once did a wrist roller once up and down, one set pronated grip, one set supinated grip, with about an 8 foot rope, so it took about 25-30 reps to roll it up, every day for a couple of years.
I used a set of steps with an open back to rest my forearms on to help to isolate them. At the time I was a teenager, and training mostly for baseball. I think this alone improved my bat speed and power and overall batting average more than any other single thing.
Since then I have had "Popeye" arms, being that my forearms look a little too big in proportion to my upper arms, but some portion of that may have been genetic anyways.