Pre-HRT I've had success with hypnosis. Close your eyes, lie back and relax. Imagine your nipples blossoming into full sized breasts. Imagine you can feel them getting larger and becoming defined. Do this for a few minutes, and you'll feel a tingling in your chest. Once you've practiced a bit, you won't need your eyes closed, and you can do it anytime you have a spare moment. Sitting at a traffic light, waiting in a Dr.'s office, etc.
I got some very modest breast enlargement that way, too.