There are these magic products that are camisole substitues. Instead of wearing an actual cami and adding bulk to clothing, these inserts clip on to the bra strap and provide cleavage coverage.
This first link used to be the best out there but they replaced the metal snaps with cheap plastic hooks that come off easily. I bought these ones and replaced the plastic hooks with the metal snappers.
These dudes have the metal snap-ons which are awesome but the material width is too narrow.