I've read of other cross-dressers managing this, so I figure it's time for me to give it a try.
You start by buying two items:
1. A push-up bra, like this one from Maidenform:
2. Push-up pads, like these:
Put on the bra, and pull your excess chest flesh (I have plenty, especially when I'm in my corset) into the cups; then insert the push-up pads between the bra and your own flesh. The theory is that it creates the illusion of cleavage. I've seen photos of other CDs that make it work. On Monday, I plan to buy the necessary "equipment" and give it a shot.