Hand man
Hold out your palm and tell someone that you have a three inch man standing there. Ask them to tap the man on the head. Then ask them to shake his little hand. Then ask them to close their eyes and poke the man up his little arse. At this point you quickly place your pursed lips where the man's arse would be, so your friend sticks his finger in your puckered, wet arsemouth.
At this point, your friend will probably open their eyes, as they weren't expecting the little invisible man to have a tangibly wet anus. You will be looking up to see their reaction, pretty much like a dog. It's difficult to know who's in the most undignified position, really.
written by an*nymo*s us*r, approved by Susan

In a different version, you stopped after the "shake the little man's hand" part, looking in shock as we exclaimed "That wasn't the man's HAND!"
This version is less likely to fail thanks to your friend's reluctance to close his eyes in the middle of what is clearly a piss-take.
written by Ka*hry* Webe*, approved by Log