Whack your forehead with the "palm side" of a handshape that is a cross
between an "A" hand and an "S" hand.
(By that I mean the fingers are tucked in as in an "S" handshape, but the
thumb is tucked to the side of the hand as in an "A" handshape instead of
wrapping around the front of the hand.)
Note: I recommend you be careful doing this sign when you are angry. (Heh.)