Good bra, "flies" First of all, you need to prevent further sagging by wearing a bra that gives you good support. The ligaments of cooper that hold up your breasts stretch with age so you need to prevent further damage to them. Doing exercises that increase your "pecs" or pectoralis major muscle will increase the muscle below your breasts. Your local gym can help you learn how to do exercises such as "flies".
None. Sagging breasts are caused by a larger amount of breast tissue and maybe weaker coopers ligaments, which serve to hold up the breast tissue. This is sort of genetic, and sort of caused by your pregnancies and life. Wear a supportive bra, if you can.