How about the iPhone picture routine, where you have some cool pics in your photo album, and you are flipping through commenting on each one with a DHV story of what you were up to - and after 15-20 photos then you "accidentally" swipe your finger and it is obviously a self shot of a huge cock. You deadpan and say something like "you didn't just see that" and put the phone back in your pocket seemingly shocked you showed her a self portrait of your cock - all the while maintaining eye contact with her. Apologize, and excuse yourself for a cigarette. Go chat up another other girl up for a while. Return later than she thinks you will. Give it a solid 20 minutes.
She won't forget.
She won't forget.