It's inevitable. Regardless of whether you are a biological or adoptive mother, or whether you are a mother at all...
You will sag. The girls head south. It's like retirees that go to Florida for warmer weather, they want to go too. Just sooner than the rest of your body.
"When the gravity pulls the breasts down, those ligaments and the skin can stretch, and so the breast then droops...Another common cause for sagging is when a woman loses weight. When you lose weight, some of that fat disappears from your breasts." ~http://www.007b.com/sagging.php
In doing my research, I found that nothing can prevent sagging. Even wearing a bra, which many women try. Of course, when a bra is on, the girls are where you want them. But as soon as you're free, they are as well to stretch as far as they can.
In fact, some research suggests that excessive bra wearing can cause MORE sagging.
"In all subjects, after 3 months of brassiere constraint, the underbust circumference was smaller but the chest circumference became enlarged, the distance between the right and left nipples became wider, and the breasts tended to hang down. This change was more marked in obese subjects with pendent breasts. And when this type of subject wore a 'well-fitted' brassiere for a long time, her breast form became developed, that is, her breasts hung down more." ~Ashizawa K, Sugane A, Gunji T Institute of Human Living Sciences, Otsuma Women's University, Tokyo, Japan: Breast Form Changes Resulting From A Certain Brassiere Journal of Hum. Ergol.(Tokyo) 1990 Jun; 19(1):53-62.
I don't know what one can do to prevent sagging. Nothing I suppose. But regardless of whether or not your chest is trying to vacation without you, be proud of your body. I'm sure it serves you well!
PS - Yes, I realize I could have gone into an entire tirade about "the girls" and "your man," but we'll save that for another day.