Okay I was reading in seventeen magazine and it is a work out sheet, you start standing up arms in the air, put your hand on the ground like the down dog yoga position (type in google images "down dog yoga position") when you get in this position walk your hands out into a push up position. Then do a pushup, after get up and start over. Do it 5 times take a short break (1 min) then do it again 12 times again.Do it in the morning and night. You can take one day a week off.
I have also heard if you come out of the shower grab or rub them 20 times. (I am not sure about it). For the time being as this will take some time wear a normal bra, get a tape (that wont hurt when taking off) and tape it from the side of one breast, to the side of the next (out side part closest to your arm pit) tape them so you get cleavage. When you put the tape on put the bra you have and that fits over top, you might need to stuff the sides with something. Toilet paper or something. Just make sure it doesnt pop out (whatever you are stuffing)
Hope this helps! more tips: how to boost your bust
PS- I used to be and A cup 2 years ago and now I am a C cup. I am 17.