A: As a method of relaxing, head-banging leaves something to be desired (once you are past the age of 18 months where it does release a parent-scaring amount of serotonin). As a method of driving nails, it also leaves something to be desired - namely, a callus thick enough to keep the skin from being damaged. In order to callous* your forehead, work your way up with a weekly regimin of chopping wood with a forehead-ax, digging with a forehead-shovel, hoeing with a forehead-hoe, and doing non-nail-involving forehead carpentry. Ted Williams used to callous his forehead by excessive forehead batting practice. Peeing on your hands, despite popular myth, will not assist in callus formation. In a matter of weeks, you’ll be able to bang your head against a wall – and hang that picture while you’re at it!
* That’s the verb form! --ed.