Answer:
A really good bra!
The tissues in the mammaries will sag and stretch over time due to gravity, lack of proper support and activities which cause the mass of the breast to shift or bounce about uncontrollably.
Fortunately this is somewhat reversible if you take care of your breasts while your body still is producing enough natural collegan to restore the tissues and tendons in the area once proper breast support has been secured.
If you are well endowed you want to ensure that the mass of weight is being supported via the shoulder straps and hooks/fasteners of the bra.
The bra should be SNUG on your ribs or will not be capable of supporting any breast weight! The bra should not however 'cut' into your skin leaving painful or uncomfortable red marks.
A good bra however will fit securely enough that at the end of a long days wear you will likely have red compression marks from the bands (if you have large breasts).
Red marks, ok. Red marks that hurt, not ok.
If you participate in regular heavy physical activity such as running, wear TWO sports Bras for added support. One over the other. Often (top heavy) female runners will wear two sports bras (sports bra have more elasticity and will work better for this purpose). One very tight one (dont be afraid to pop out a little over the top or on the sides) for primary support, and a second more generalized sports bra (a pull over one v/s one with hooks/clasps) to smooth out the tighter bra and secure everything in place. Dont wear anything that will restrict your breathing, but for activity purposes you want something that holds up while you bounce.
Also to prevent saggy boobs a simple sports bra or sleeping bra is helpful. They prevent the breast tissue from sliding to the side if you sleep on your back, they also help to hold your breasts in place if you sleep on your side.... All of which will alleviate the stretching and pulling of the tissues which cause droopage.
You dont need to wear your boobs up under your chin all the time, but the more they drag the more you will sag. Make sure your bras fit properly, and support the weight of your breasts.
Wear a good bra that keeps them in place
No.
Tight
no
If your breasts got saggy then you will need surgery
No, aging and pregnancy can change the shape and condition of breasts and the skin.
Your breasts get saggy, and they detieriate
Because they are VERY fat and saggy, ok?
There is no record for the World's Saggiest Breasts.
so your breasts don't get really saggy when you're older.
Its got a really ugly face and saggy breasts
There really is nothing that can be done except for surgery.