Ladies, fret as we may, some of us have (or will have) saggier breasts than others. Some point their fingers at gravity and others at the bras we wear, but the bigger culprit is your genes. Just as genes determine the colour of our eyes and hair, not to mention the size of our breasts, genes are the big determinant of whether our breasts are more prone to droop over time (referred to as ptosis).
According to Stuff Mum Never Told You, there are a myriad of factors at play here, but many point back to genes, such as your breast's skin elasticity, tissue density and size, as well as age. Significant weight changes, pregnancy and other behavioural factors do play a role, but the big takeaway is, basically, gravity plays only one part. Check the video (above, or linked below) for more details.
Saggy Boob Science [Stuff Mum Never Told You - HowStuffWorks]