I've always had a fairly large chest that was perky. Since I've gained weight and gone up in size (from 36c to a 36dd) The perkiness is going, going, going, almost gone!
What specific chest exercises would work best to keep them from dropping further?
Loosing weight would help since they got bigger and less perky with your weight gain. Any chest exercises help build muscle behind the boobs, which helps them look perkier. Push ups, chest press, pec flies are good. Maybe you're hoping to keep the size, but I don't think that muscle behind a 36dd is going to help much.
-------------- Even if you are on the right track, you will get run over if you just sit there.