Have sex with your girl in the "doggy style" position in front of a window. On the "out stroke" pull completely out and have a friend (or if your from the south, a relative) fill in for you. Hopefully she will never realize the switch. At this point, run outside to the window, stand in front of it and wave at your girl while she is still getting nailed by who she thinks is you.
i david blained that chick and she never knew what happened