You can also make some pretty nifty corsets yourself out of duct tape, which sounds silly but with all the colors they have it can look really amazing.
I personally use a nice push up from VS if I want a 'casual cleavage' look. But for amazing cleavage I use a push up bra with a low cut spaghetti strap tank with a 'built in bra' which is usually just a non-stretch piece of fabric in a size smaller then I should be wearing. Then I layer whatever else on top of it.
You likely could use a low cut sports bra for the same effect if you want to cover up less.
Corset duct tape making:
You technically can place other fabric over it, but it's sure a lot cheaper then any one you would buy.