For sleeker looking arms, try exercises designed to improve your posture. Most posture exercises work all the major muscles in the arm, allowing them to be strengthened and toned. Try extending your arms until they're at shoulder height, with your elbows bent and palms up. Hold this position for at least 5 seconds, then release and repeat. Performing this exercise at least 3 times weekly should have you standing straighter and leave your arms looking great.