There is an exercise called a side plank found in yoga called a side plank. It is great for your core as well as your arms. Begin by lying on your left side with your legs straight, your feet flat and your torso should be propped up by your forearm and your elbow. Now as you push against the floor with your feet (your right foot should be on top of your left foot) and your elbow and forearm, lift your body off the floor. Your elbow, forearm and foot should be the only parts touching the floor. Now hold this position for 20 seconds and repeat for 3 sets on each side.