If you insist on arm wrestling, learn to controll your oponents wrist by articulation of their thumb in your grip, you wont need so much effort then.

Also don't try to push his arm down which far too many people do. Use your arm's natural range of motion and pull his arm towards you like you are doing a bicep curl. You'll have more strength and less stress on this area. Plus when his arm is extended he will have less strength to stop your from putting the smack down on his ass.