One of my co-workers mentioned to me that if I sit down too much my ass will get flat, at the time I took it as a joke…
But now that I think of it seriously, it’s not as crazy as it sounds, besides, a big proportion of people I see here have big and flat asses…meaning large but flat, not well rounded…
So, what’s the straight dope on flat asses? It’s really important for me since I want to keep my rear end’s good looks! 
I seriously doubt it. Large but flat asses are (I think) due to a lot of fat and little muscle. If you wanna keep your ass looking good, build the muscle in there, and try to keep your body fat % reasonable (6-11% for a male, 15-20% for a female, no cite, just numbers I hear a lot).
What ultrafilter said. Your ass will get out of shape if you sit on it a lot. Not because you’re somehow pressing it out of form, but because, if you’re sitting a lot, you’re not getting much excercise.