All breasts fall to the side when lying down-simple gravity. Sagging is when they fall lower than they used to when standing without a bra on.
I've heard of people doing upper body workout to strengthen the chest muscles which apparently can help keep them higher.
But it sounds like you're unhappy with how you look. So why not explore your options and consider some cosmetic surgery? Or you're going to have to come to terms with the fact that you can't do the miraculous task of carrying, birthing and feeding children without it changing your body.
