So the French have conducted a study that concludes bras cause saggy boobs!  No shit, Sherlock!  I've noted for years that my belt causes my saggy gut!   Jest sayin'....   My own "study" of breasts over the years, admittedly with a limited sampling confined to some of the most "talented" ladies in the hobby, concludes that a vigorous regimen of hand massage does wonders for keeping those puppies firm (well, I suppose utilizing a reputable cosmetic surgeon doesn't hurt, either! )

But really, hands-on massaging is the key.  No different, really, than the "hands-on" approach to a firm penis.  Although some studies have concluded oral stimulation has similar results.  

I suggest further research is warranted.  I've applied for federal funds....waiting on a response.

The accuracy of any statistical study of a population increases with sample size.  Since four hands can undoubtly examine a larger number of breasts than two, I volunteer to assist you.