I used to have the same problem. I was the typical flat chested ballerina. About as busty as a prepubescent boy. ;]
Over the course of about eighteen months, however, I went from an A all the way up to a C, while still maintaining my slender frame. And no, I am not pushing any weird medications. I am not sure at all what brought on the breast increase, but this is my best guess:
1. Diet change a. HUGE increase in dairy consumption (a lot more milk, cheese, yogurt etc.) b. More meat (mainly an increase in red meats, but I still ate a decent amount of poultry) c. More fruit (supposedly papaya helps increase breast size)
2. Exercise change Originally I had been dancing 20+ hours a week, but when I started college I had to reduce to just a few hours. I noticed that my breasts began to grow when I wasn't exercising so often. And please do not use that as an excuse to not exercise, because that it still very important. I just think sometimes people can go too far with conditioning their bodies, and that can cause them to grow (or not grow) in ways that may not be completely natural.
But as far as your current "problem" (I hesitate to call it a problem because in my little world of professional ballet, flat chests are the absolute ideal) I would say just be comfortable with it. If your flatness makes you uncomfortable, then draw attention away from that area by wearing clothes that emphasize your good qualities. Long slender neck? Mile high legs? Tiny waist? Dress to emphasize those areas instead. And if a guy considers your breasts to be a deal breaker, then he is obviously not the guy for you. [=