Pick out the cup-size, the sorts of activities you do or plan on doing, and the page will give you examples of the movement of your breasts during activity. If a sportsbra isn't doing a good job of keep breasts from bouncing around, obviously the bra isn't good enough. A proper bra will help prevent potential sagging and so on.
Posted in a_better_body, body_positive, bodygoddesses, health_fitness
[edit: Warning, there is nudity in flash demonstration.]