If you have one breast that's slightly larger/smaller than the other, here are some tips for you: buy bras that are contoured. (When searching for this contour bra, always remember to fit the larger breast first)! After a few wears and washes, the cups begin to form to the natural shape of your smaller breast, without the noticeable gaps of a cup too big! Two brands I recommend: Calvin Klein 365 Microfiber Stretch Contour Bra style, #D2894 and Victoria's Secret Infinity Edge Shaping Demi Bra, style, #212589.
P.S., I has been said that molded cups can provide the same benefit. In my experience, it is true to some degree, but I believe contoured cups work better!