Well, if you want something more substantial under the bottom you could wear a pair of "lineless" underwear, you know the kind that keeps your panties from showing through a clingy dress? They come in different panty cuts, little shorts or hiphuggers that fit snugly, and a bunch of skintones.
Under the top I strongly recommend adhesive bra cups. They're strapless, backless, skin-tone, supportive for most bust sizes, and can be trimmed so you don't see them in that dramatic neckline or that open strip under your arm. And that way even if something shifts in an unfortunate direction you won't flash someone. ^_^' There are also ones that are an actual re-usable cup , but you may not be able to hide it under that costume.
If you are worried about the costume shifting around, there is a special skin-and-fabric-safe double-sided tape for that, I just can't remember the name...
JEN0VA: The Mother of all evil. And villains in tight leather.
Larsa Ferrinas Solidor: Can peg a gnat with a Hi-Potion at 50 paces.