When you pop someone in the back of the head, how do you protect your knuckles from being crushed?

My pinkie knuckle on my right hand always hurts me, ever since I knocked this one guy out in a bar by t him in the back of the head with my fist. (Now he looks all strung out) I told him his head makes my knuckle hurt, and he said, "I hope it hurts you all your life, @sshole." Nice. But, how do I get it to stop hurting.