Sit with your knees 2ft apart and feet flat. Hold a dumbbell in each hand, palms up, and lean forwards so your forearms rest on your thighs and wrists hang over your knees. Using your wrists only, curl the weights up as high as possible. Now rotate to a palms-down position and bend your wrists up. Do 12-15 each way.
Sitting on the edge of a bench, hold a dumbbell in each hand, letting your arms hang by your sides. Keep your back straight and curl the weights up until your thumbs are near your shoulders. Squeeze, then lower. Now rotate your wrists so your palms face backwards and curl again. Do 8-12 reps, alternating grips.