Much of breast tissue is fatty tissue. If you are overweight, losing weight may help as well. It is important to participate in aerobic activity in addition to strength training and to follow a healthy diet as described in Strong Women Stay Slim.
Some women see improvements when they tone the chest muscles that are just below the breast tissue. The upper body exercises in the StrongWomen programs will help. Be sure to wear a comfortable, supportive bra when you exercise to minimize bouncing and to avoid stretching the breast tissue.