They sell these inserts nicknamed "chicken cutlets." You can slip them into a padded bra, and they will really help give you a little extra oomph.
Strategically placing a little bit of highlight to the top portion of your breast can also give the illusion of having a fuller-looking bustline.
Works in Progress:
Fujimoto - Ponyo on the Cliff by the Sea
Mewtwo Gijinka - Pokemon
Convention Schedule 2014: