First of all, lets go into the reasons of "Why the sagging"? Some of us tend to believe that the breasts start sagging after pregnancy or because of breastfeeding. Sagging breasts are a common result of aging and gravity. It is determined by skin elasticity, genetics, and weight gain as well. I should mention that large breasts will sag easier obviously because gravity is pulling them down more. So in essense a woman can have sagging breasts regardless of pregnancy or breastfeeding.
Having said that, lets talk about the idea of making them look bigger. Your breasts are actually made up of fatty tissue, so their size depends upon how much fat you tend to store there, which calls in factors like your age, your health, your genetics, and your weight. The muscles that reside directly underneath your breast are called the pectorals. They are partly responsible for the "perkiness" of your breasts. So, you cannot do anything about the size of your breasts, unless you gain weight or have surgery. You can, however, make them appear bigger, firmer and more shapely by developing your pectoral, or chest, muscles.
So what can we do to keep our breasts perky and firm?
DO WEIGHT TRAINING EXERCISES
AVOID EXCESSIVE SUN EXPOSURE
AVOID SMOKING
CONCENTRATE ON KEEPING A STRAIGHT POSTURE
WEAR A BRA THAT HAS GOOD SUPPORT AND FITS
What exercises can help in achieving great looking breasts?
Incline Push-Up
Lie face down with hands on a secure bench, desk, or chair. Place hands slightly wider than shoulder-width apart, with feet hip-width apart and toes on floor
Lower your body so that your chest is 4-8 inches from the bench
Return to the starting position by extending at the elbows and pushing the body up
Try 2 sets of 10 reps for starters
Dumbbell Bench Press
Lie back onto a flat bench with a dumbbell in each hand
Starting Position: Lie onto your back and bring the dumbbells to your shoulders. Press the dumbbells up directly above your chest with palms facing forward
Lower the dumbbells slowly, keeping your elbows pointed out
Let your upper arms go parallel to slightly parallel to the ground before returning to the starting position
To end the exercise, place the dumbbells on onto your thighs or at sides
Try 2 sets of 10 reps for starters
Incline Dumbbell Chest Fly
Lie back onto an incline bench with a dumbbell in each hand
Starting Position: Lie onto your back and bring the dumbbells to your shoulders. Press the dumbbells up directly above the chest with the dumbbells almost touching and facing each other
Keep the elbows slightly bent, lower the dumbbells out and away from each other in an arching motion with hands aligned with the upper chest region
To end the exercise, place the dumbbells on shoulders, then onto your thighs or at sides
Try 2 sets of 10 reps for starters
For more exercises that target the chest area, read my article on Toning And Firming Part 2-Monday's Workout.
Tips: Same as the 
Tips: Position your feet 2-4 inches wider than your shoulders. Hold one dumbbell in front of your body. Point your toes out at a 45 degree angle. While standing upright, keep your knees slightly bent with your hips in a neutral position. Don't arch your lower back. To begin the exercise, start by moving your hips back like you were going to sit in a chair. Then, bend your knees letting them follow the exact angle of your toes. Lower your body until your thighs are parallel to the floor or to a position that feels comfortable to you. Exhale and return to the starting position. Remember to maintain good posture with your chest up. Also, keep your feet flat on the ground throughout the movement.
Tips: Hold a dumbbell in each hand and pull your shoulders back. Lift your chest up and look straight ahead. Position your right leg forward in a long stride. Your foot should be far enough in front of you so that when you bend your right knee, your thigh and lower leg form a right angle. Slowly bend your knees, lowering your hips so your rear knee just clears the floor. Pause briefly in this position, then slowly straighten your legs and raise your body back up to a standing position. Complete a full set, then switch legs and repeat, or alternate legs for each rep. Make sure your knee does NOT travel past your toes in the down position!
Tips: On a step or platform, (or on your bed if you train at home) lie facedown with hips on the edge of the step, legs straight with toes resting lightly on the floor. (If you are using a bed, your legs will be off the edge and your feet high off the floor.) Squeeze your glutes and hamstrings and straighten the legs until they are level with the hips. Lift one leg higher then the other and alternate. Move each leg as though you are doing a flutter kick in water. Try doing 3 sets of 20 repetitions on each leg.
Tips: Lying on your back with your knees bent, lift your hips off the floor while keeping your back straight. Remember to keep your glutes tight!
Tips: Lie flat on your back with your feet flat on the ground, or resting on a bench with your knees bent at a 90 degree angle. If you are resting your feet on a bench, place them three to four inches apart and point your toes inward so they touch. Place your hands lightly on either side of your head keeping your elbows in. Don't lock your fingers behind your head! Push the small of your back down in the floor to isolate your ab muscles. Begin to roll your shoulders off the floor. Continue to push down as hard as you can with your lower back. Your shoulders should come up off the floor only about four inches, and your lower back should remain on the floor. Focus on slow, controlled movement - don't cheat yourself by using momentum!
Tips: Using a decline bench, position yourself with your feet locked in at the top. Your upper body should be raised off the bench so that you have to contract your abs just to stay in place. Place your hands on each side of your head, over your heads. Don't lock your fingers! Raise your body slowly while you contract your abs. Crunch up until your elbows are on either side of your thighs. Hold and flex your abs, then slowly lower your body back to the starting position. Don't lower your body all the way down the to bench! If you can, hold a weight plate across your chest for added resistance.
Tips: Lie on your back on a decline bench and hold on to the top of the bench with both hands. Don't let your body slip down from this position. Hold your legs parallel to the floor using your abs to hold them there. Keep your knees and feet together. Slowly contract your abs, focusing on bring your pelvis up and in towards your chest Let your abs do the work! Slowly lower your pelvis to the starting position, keeping constant tension on the abs. Don't arch your back inwards while you are at the down position!
Tips: Secure your feet either by placing them under something that won't move or by having a partner hold them. Start in the position shown above, leaning slightly back and clasping your hands in front of you. Moving only at the trunk, rotate to one side. At the end of your range of motion, quickly reverse the movement and rotate to the opposite side. Repeat in a rapid fashion for the full number of reps. You can also hold a weight or medicine ball to increase the difficulty.
Tips: Works your lower abs. Lie on a flat bench with your legs off the end. Place your hands under your butt with your palms down. Put your legs straight out. Bend your knees, pulling your upper thighs into your midsection. Return to the starting position. You can hold a dumbbell between your legs to make it harder.
Tips: Works your lower abs. Place a light weight between your feet if you can. Lie flat on a bench with your legs off the end. Place your hands under your butt with your palms down. Keep your legs as straight as possible and your knees locked. Raise your legs as high as possible. Lower legs back down as far as they can go.
