There isn't a spell that I know of that could aid you with this, unfortunately... but there is an exercise. It helps by strengthening the muscles under the breast tissue, thereby "pushing out" what you already have.
...But before I get into all that, trust me- having been on both ends of the spectrum (I never even had enough to cause even a movement
let alone any bounce or shadow of cleavage until after I had and nursed my daughter... then I jumped up 3 cup sizes--) a smaller bust, while it can leave "something to be desired", is actually a lot more comfortable to wear than having larger breasts is. We smaller sized women can lay on our stomachs with no pain or issue, we can jump up and down or go running and vigorously exercise without feeling that our skin is going to tear off of our torso... and we never ever have back problems as a result of additional weight from a large bust. Even though you may feel small and think you don't like it... there is a greater benefit gained from having a smaller bust. Plus, there's gravity: if there isn't much there to start with, it stays where it belongs. When there's a lot of breast tissue, over time it begins to sink down and droop. You've seen those older ladies who could literally
"tuck them in their belt" right? Yet another problem you won't have.
(And as my husband always said, "A handful is all you need")
OK- all that said, here's the exercise: (from http://www.top10herbal.com/exercises.htm
"Targeted Chest Flies : This exercise is really just a modified and slightly more targeted exercise which specifically targets the pectorals at a much lighter rate, gently guiding them to increased firmness and a bigger look.
Take a very light pair of hand weights, 5-10 pounds, and lie on the floor, chest up. Now, raise you hands with the weights gripped in them lightly, directly out in front of your face, so they are perpendicular with your chest. Take them out to the sides of your body, not moving them more than a couple of inches, concentrating on gently squeezing the sides of your breasts.
This targets your lateral bust supporting muscles gently, since it is such a gradual movement, and is very slow and deliberate. Do three sets and ten reps of these, slowly and with focus on the sides of the pectorals.
Mini Desk Workout : This is especially for those of you that work at desks for most of your day. You can take the palms of your hands and put them together, concentrating on squeezing your bust muscles together gently. This should feel pretty natural, as these are the muscles you must use in order to feel the opposition in the palms.
Spin Doctor : This is one I named especially for the silly look you have when doing this exercise, which blasts the muscles which support the breasts, and is an excellent exercise to increase bust frimness. With this exercise, you will be standing. Put your hands out directly in front of your body, with your fingers all flat and pointed out to the distance, as if you are reaching. Now, move your hands in toward each other, as if your thumbs will touch, but slide one hand under the other. Repeat this, with one hand going over the other alternating turns. You may pick the speed up but will want to start this exercise slow so you do not accidentally get overzealous and pull a muscle.
Standard push-ups are also a good exercise for improving the firmness of your breasts. By strengthening the chest muscles, your breasts will be lifted."