You have to take care of your overall health eat fresh healthy food with lots of proteins iron and calcium do regular exercises targeting your breast muscles if you do not know them then learn from some expert do regular message of your breast atleast once a day wear a correct size bra with comfortablly snug fitting keeping proper tension in your straps.
Take 2 ice cubes and massage them in circular motions around each of your breasts for 1 minute only. Dry the breasts with a soft towel and immediately put on a proper fitting bra. Stay in a reclining position for 30 minutes. Do this at regular intervals throughout the day.
You can also do some breasts lifting exercises for that purpose.