My are very large. How do I stop them from sagging?

I'm 19 years old. I already think there too big. But I don't want to go get any kind of surgery and I do not just want them to be perky in a bra. I want them perky when I'm dressed AND . I know my weight might have a lot to do with it And I've lost weight, but they are still large and saggy. I want them to perk and be round. How do I get them that way naturally?