Underwires, boning, and padding all add fullness to your chest. Just be sure that any padding is subtle and, once wet, doesn't show through.
Ruffles, like shirring, can also be a plus, since they can make any area look bigger. (Just as you might not want them along your hips, you will want them fluffing up a smaller bustline.)
Also consider the art of distraction: Try a top with a pattern, polka dots or embellishments.
Finally, make sure the colors of any separates work together in your favor—a bolder color up top and a darker one below will add the right oomph.