LADIES question. Do your sag??

I'm only 18 year olds and I have s that look like my moms. Both of my sisters are uplifted, perked up like most of my friends. I know, I know, it's not a BIG deal, but HEY. I don't want to always go to Victorias Secret to get those GREAT push up bras. My are not touching my stomach, but they sag and are droopy. My mom says that it's because when I was going through puberty i used to push my down cuz I was a tomboy and hated the thought of growing ies haha I was just WONDERING if theres any NATURAL way of getting a nice lift. Rumors i heard include exercise. But I used to run cross country, Oh my gosh, that never helped. Push-ups? Sit-ups? Have you guys done anything similar to get lift and seen results. Please: serious inquiries only.