Get a set of weights and start with the lighter one and as long as you are prepared use a heavier one and keep increasing the weight until you consider it's enough weight. Do the up and down movements without leaving the tension zone, which means you can never open the angle of the movement of your arm until you reach a position where you feel comfortable with.
Details: Practice a good posture or you may be injuring your back. Don't use weights that are too heavy for your muscles or to your bones because that may be dangerous to your body. Do the most repetitions you can and try other recommended variations of this exercise. Warm up your fingers, hands and arms joints if you think it's necessary.