Heavily padded pushup bra. They're available in strapless too. I personally like the "modular strap" kind that you can wear with or without straps, and change the look of the straps depending on what you wear. I got mine at Sears.
Costume Con - Kili in a dress
Atomic Lollipop - Come visit me at the Make-Your-Own-Button free craft booth!
Polar Chill - check out my costume DIY panel and workshop
FanExpo - Iron Man Mark VII version 2, Pacific Rim Jaeger pilot?
FanExpo - fem!Iron Man Mark VII - The Iron Man Armor Project
Atomic Lollipop - Vanellope von Schweetz; raver!Sailor Mars