The breast is made up of glandular tissue and fat, held in position only by your skin. While skin is elastic, it's subject to the forces of gravity and the weight of your breasts. The stress is amplified when you add bounce, whether from fitness or just daily motion.
To maintain the shape and firmness of your breasts, wear properly-fitting support every day. When exercising, wear sports bras that provide support relative to the activity. Don't make exceptions for backless, strapless tops: choose appropriate foundations to wear under them, or if you must, go braless for only a few hours.