@Magifox: Just really quickly: did you shift your breasts at all? For example, putting one sports bra on, then shifting both breasts up and to the side, then putting the other on? I know when I wear a sports bra just for being active, all it does is compress my breasts towards the middle slightly. It's fine for being active, but not for crossplaying... if I were going to crossplay with a sports bra I'd probably shift them around a bit and see how well that worked.
I'm not sure if you tried that or not, but if you already did, then try some other things as well, like different sizes, like Kelley suggested. :>