I worked for bra companies, and so I know a little more than average about breasts, while not being a doctor. First, the connective tissues in the breast, once stretched can not be repaired. We can exercise a muscle back into shape, and indeed there are some supportive muscles in the breast, but mostly saggy deflated breast are not that way because of the muscles, but because those tissues and skin are stretched. So having smaller breasts generally helps, staying constant in your weight helps, wearing a supportive bra helps a lot.
About bras, the wire should be laying on the chest wall, not digging into your breast. Most people wear the wrong size, and the mistake is typically they wear the wrong back size, which is the number, not letter of the bra. If when you raise your hands above your head and then bring them back down you have to adjust your bra, your bra back is too big. If your straps constantly fall down, your back size is too big. Many people are walking around in 36B and they are actually 34C.