If you're busty you probably need to go to a specialty shop to get some good quality sports bras. The S-M-L-XL variety aren't going to do the job if you're doing high impact activity. If you're not jumping around then you can probably get away with it.

Whether it's fortunate or unfortunate I don't know, but I don't have to worry about that anymore. $5 sports bras from five below do the trick for me now...