I don't workout at the gym, but I do play football sometimes, and for that I wear a compression shirt from UnderArmour (wearing it under your binder can help stop the binder chafing when you exercise), followed by a fairly tight tri-top binder, and then a looser t-shirt or sports shirt. I never get completely flat, but I feel fairly comfortable. I find it's the upper portion of my chest (the bit of moobs at armpit sort of level) that needs the most binding, so as long as that's compressed, nothing wobbles around and I'm fine!
