Once or twice a week, at the end of a training session, grab a pair of fairly heavy dumbbells - I use 70's or 80's - and time yourself on how long you can hold them. Start with a weight you can hold for approx. 60 seconds. Keep trying to beat your previous time, working up to about 90 seconds. Then, next time pick up a heavier set of dumbbells and start over.
Do this for a month or two and I GUARANTEE your grip strength & forearm size will improve!