I would think doing butt cheek-clenching exercises would help, I definitely squeezed a couple out reading this thread.

Seriously though, I think OP is referring to something I do sometimes on really steep climbs to get more power, kind of a desperation move to avoid standing up. I've never done it on the flats. However I have noticed that certain bikes I own give me the ability to "sprint" from a seated position. My Raleigh Competition is like this, it's one more great thing about that bike, but I'm not sure what precisely it is that gives a bike this quality. But that's fodder for another thread.