Also sleep with a comfortable bra/sport bra and maybe invest in some skin firming creams for that area. Ultimately the yoga will be your best bet!
Exercise also with yoga . My favorite one for keeping the boobs healthy looking is called Cobra Pose. Look for a photo of it. Start by lying on your stomach. Place your palms against the floor right below your shoulders. Now arch your back all the way up with you pelvis still against the floor. Tilt the head back and feel the nice stretch in your torso. Archer and the Bow are also good poses. For a real challenge, there is Camel. Also sleep with a c
Definitely do what she said!^^ exercise on the top part of your body helps a lot! Especially with larger boobs. Bench pressing, medicine ball push ups, etc!
Larger breasts have the tendency to do that, unfortunately. You can try exercises that strengthen your chest muscles as that can help somewhat, but other than that, there's not too much.