You can't firm up the ligaments that support your breasts. But by toning the muscles that support your breasts, you can give yourself a mild lift and help prevent future drooping.
Focus on the pectorialis major, the fan-shaped muscle that stretches from your clavicle down to your ribs and, in women, lies underneath the breasts. And don't worry about looking too bulky—you'd have to be taking steroids (or genetically blessed with incredible muscle tone) to build that much mass.
For the most effective workouts, consider personal training sessions with a professional.
Dumbbell Chest Press
Lie on your back on a a stability ball, flat bench or aerobics step, with feet flat on the floor. Hold the dumbbells at shoulder-level and contract your abs. Press up while exhaling until the weights are directly above your chest. Your wrists should face forward. Bending your elbows, slowly lower the weights—this forces your muscles to work harder.
Start with 5- to 8-pound weights, and use heavier weight as you grow comfortable. Aim for three sets of 12.
Push-ups challenge your entire body, especially your chest muscles. If you don't have much upper body strength, start doing them on an incline, say, against the kitchen counter or a park bench, then work your way up to full push-ups on the floor.
Start with your arms fully extended and your abs contracted, maintaining a straight line from shoulders to heels. Bend your arms and lower your body until your chest is just a few inches away from your hands. Slowly return to the starting position. Repeat.
Lying on a bench or stability ball (flat or on an incline), start with dumbbells by your shoulder, then press them above your chest, with wrists facing each other. With slightly bent elbows, lower the dumbells out toward the sides and away from each other. When the weights are at shoulder level, return slowly to the starting position.
A regular yoga practice will improve your posture, which can help your breasts sit higher on your chest. Even better, you can work the chest muscles by striking the inversions and chest-opening poses frequently found in vinyasa, Bikram, power and ashtanga yoga.
Wheel or Upward Bow
Lying on your back, bend your knees and bring heels to sit bones. Bend your elbows and place palms on the floor by your neck, fingers pointing toward your feet. Press your feet into the floor and contract then lift buttocks off the floor. Take a few breaths, then press hands into the floor and lift up onto the crown of your head. With another exhalation, straighten your arms and lift your head off the floor. Stay in the post for five to 10 seconds.