[Samantha Rogers]

Well, my first thought is that we cannot defy gravity any longer than can genetic women. If they get big enough, eventually they will sag. I don't think it is realistic to suppose that a even a relentless diet of PM, regardless of dosage, will be enough to keep them firm and/or growing forever. Sigh...
As far as a bra goes, I am no help at all as I have never worn them, except for brief forays into complete femme, and then with the aid of enhancers. I cannot comment on the results of regular use. I can say, if preventing sagging breasts is the goal it certainly has not helped my wonderful wife. Of course, hers are well beyond the point where nothing short of a complete break down in the laws of physics would prevent sagging.
On the other hand, I believe I have read that regular bra usage may actually contribute to sag by preventing the growth and strengthening of good elasticity. I believe a search of the various forums here will provide more than you can possibly ever read, if you want to go that route.
Hugs