Hmm... never tried, but maybe some combination of sports bra AND binding.
I have seen ace bandages used, that might work.
But keep in mind that binding may add to the size too... probably want to focus on finding a way to push the fat/tissue of the breasts to a less conspicuous area, maybe the back? No idea.
It depends on how big they are. If yours are small, like Nighty's, you don't really need anything. But if they're ginormous melons, you prolly shouldn't. It could cause some breathing problems. Try using bandages or a really tight sports bra. Or, use a small t-shirt. Just make sure you don't suffocate!
I feel your pain! I met a female cosplayer at Anime North who was an 34/36 F and she managed to get them to look like pecs with the right binding techniques. It is possible, but I can't remember what exactly she said. D:
Maybe check out the following links
http://www.youtube.com/watch?v=hBqzMCcuH-g
http://www.youtube.com/watch?v=J5ZcwZCVKIE&feature=channel_video_title
I'm pretty flat, so wearing my regular sports bra usually does the trick for me. ^^; I searched around, and this thread has a lot of helpful methods and tips!