As a guy, this is second hand advice. I know a girl that is no less than a DD. She wears a tight, full cup bra that changes her shape from the classic boob drape to something more like a cantaloupe cut in half, flattening and spreading her out, and with near zero movement when she walks. Once your figure and your weight settle after age 25 or so, you may either have learned to love your girls (many men will) or you may contemplate reduction surgery.
How dare you want to make your breasts look smaller, and you hate them? I think you need to realize that women like you make the world a better place for all men. Women like you make me enjoy waking up in the morning...show those beautiful big breast off, don't hide them. Get a pushup bra, low cut shirt, and some sparkles...you'll get any man you want, including me. You go girl.
What kind of sports bra have you used? They probably won't work if they're the ones from walmart :/ The ones from actual sports stores does the trick and/or competitive racing swimsuits. But those tend to be about $40 or more.
I have to wear something to make mine flat when I work out because I hate the feeling of them bouncing.
I'd say something with great support and something exactly for your size. If you try to put a smaller bra on it will make them spill out. I'd go and get a bra fitting, to help get the perfect bra. That will help lots! If you are still worried about them, I know a lot of baggier shirts are in style that might help cover them if that's what you want to do instead.
U can't really make them flatter but make sure they are 'full coverage' one and no padding. You also can make them look smaller with baggier tops that don't tighten at the bust.
a sports bra would push them down
Full cup bras - get it sized by a professional
Also watch what clothing you wear. It really depends on your build, but higher neck/round neck tops tend to make big boobs look bigger. V-neck is the most flattering. Or you can wear strategically loose tops (but not potato-sack-ish).
I know what you mean, though. I was a D cup by the time I was 16 and an E(DDD) once I was 18. Not only do they hurt my back, but I look like a slut no matter what I wear. They also make me look out of proportion cos I got fricking slim legs. Sigh. You just gotta wear them with pride!