Wrist curls and hammer curls Dave

Sit onthe edge of the bench with a light barbell(probably not an olympic 20Kg one) with your hands close together. Rest your forearms on your thighs so your hands hang over the end of your knees(keep your legs close together too). Use an overhand grip for the top of your forearms and an underhand one for the underneath.

Let the weight hang down and then use your forearms to lift it back up. I do sets of 20 reps and really make them burn. Make sure your elbows stay on your thighs

Doing hammer curls also hits the top of the forearms where it joins the bicep. You can also do barbell curls with an overhand grip, keeping your elbows close to your sides.