Thick bar DB or barbell holds have dramatically increased my grip strength.
If you don't have a thick bar, you can wrap a neck pad or towel around a barbell to increase the circumference of the bar.
Load up the bar and hold it. I personally like to hold for a maximum of 30 seconds. Increase the weight when you reach your time limit.
I would perform this exercise at the end of your workout or on off days, to ensure you're not fatiguing the forearms prior to other exercises.