First of all you need to have a mirror directly in front of your bed. Then, while you are POUNDING your girlfriend doggy style while facing that mirror you quickly pull your penis out and jam it in her anus as hard as you can while placing both hands behind her head with your fingers spread out. Look in the mirror at this exact moment and your girlfriend will turn into the fabled "Scary Moose" with sound effects and all. Find a scary moose and you will almost certainly lose a girlfriend!!!
I turned my girlfriend into a scary moose last night and haven't seen her sitting down since... come to think of it, I haven't seen her AT ALL since!!!