The diagrams above show breast construction and lymph fluid flow paths. The 1st diagram shows the Coopers ligaments that support the breasts and gives them their shape. Wearing a bra constantly causes the ligaments to atrophy and stop working and also prevents their natural movement which stimulates blood flow and lymph flow.
Sagging breasts are also a concern for many these days. There is good news and bad.
Firstly, gravity is the major cause of sagging and so don't expect any miracle cure.
Secondly a major cause seems to be the habit of wearing bras all day everyday. Bras lend support to breasts and thereby remove the need for the natural supporting system to function.(Coopers ligaments) It atrophies and stops working! So not wearing a bra so much may actually make them sag less. See www.breastnotes.com for an in depth Doctor's opinion.
A recent survey on breast sag rates shows that breastfeeding has no great effect on sag rates but that smoking does as it affects the elastin in the skin.
Bra wearing has only been popular in relatively recent times and only in western affluent countries, the same places where breast cancer rates are highest. It seems that bras may be a major cause of poor breast health and not wearing one may be a better choice. Some wear one to make their breasts perkier and more attractive (to who?) while others wear one to reduce their noticeability (nipple protrusion) I recommend no bra to retain their natural shape and wear a camisole for an extra layer of fabric. I personally know a woman who having breastfed two children over 25 years ago still has perky breasts, like they were 25 years ago, and has never worn a bra a day in her life.
This page has more explanantion about the relationship between bras and breast cancer.