A book says:
Because stoping the function of milk secretion, therefore 乳腺退化, 腺泡破裂, 腺管萎縮, so it's normal that it happens.
Method:
1) breast feed is limited to 1-2 years
2) do 健美操, help 胸部肌肉發達
3) breast massage to help blood circulation
4) choose suitable bra, don't be too tight, thicker at the bottom part to have better support
5) sufficient nutrient