How about next time you can shoot her, get her to do a breast expansion video. In the video, she could feel really hungry so she pigs out on pizzas,other take-out, and desserts. She shows you the food and tells you she is going to eat it all, and instructing you to jack off. After a while of eating, her boobs start to grow because that's where all the fat is going. She loves it so she eats more and more, as her boobs continue to grow, still telling you how to jack off. As she could be wearing a sweater with buttons, or a shirt with buttons(or something with buttons) she can feel that its going to bust out. So she tells you to blow your load all over her expanding tits.
Just an idea.