Breasts can sag with weight loss because the breasts are covered with a layer of fat, and when the fat shrinks, they may droop.  But at your young age, some of the skin’s elastic tissue might snap back.  Meanwhile, to make your breasts look perkier, maintain good posture and wear a bra that fits correctly. You can also do exercises to build the chest muscles that support the breasts as shown by the Guide to Exercise at About.com. As a last resort, some women opt for breast-lift surgery with areola repositioning.