Choose a heavy one shaped like (a) your first ride in a car or (b) the Hitchcock leg-of-lamb, served at dinner to the unsuspecting detective. Or a light objet d'art, (c) an ice cube in whose reflection is suggested the history of the subconscious.
Now choose a forehead, yours to be exact. Where your hand intersects the mirror-image forehead, strike the blow. If (a) has been your instrument, you will feel run over but sincere; if you chose (b) you will feel theatrical. (C) as you know, was Freud's preference in moments of despair.
If none of these objects appeal to you, consider the following technique: Serve as executor to a millionaire-idiot. He'll gladly leave you trifles and before you know, you'll be the owner of possibilities: a Max Ernst bed equipped with guillotine, a Picasso whose lips are razorblades, a Goya with personal angels, black and familiar.