If You're Small-Chested
Look for: A ruched bodice. Extra fabric up top will help fill out your upper body and create the illusion of curves. Lightly padded halter styles will also do the trick.
Keep in mind: The right bra will always provide a nice boost, but as many wedding dresses are strapless or backless, your undergarment options may be fairly limited. Instead, try self-adhesive silicone bra cups by NuBra (available at nubra.com).