I started doing this when I watched L.A. in the last TdF. It is actually a little easier because you're really pushing with your glutteal muscles - a larger muscle than your quads. You have to keep your torso long, though, so that you can expand your lungs. If you're hunched over, you can't breathe as easily.

I will sit & spin up long hills, but if I need to pop over, or if I get tired of sitting, I'll stand, either in the drops or the hoods. I don't rock much because it seems like wasted energy to me. I concentrate on pushing forward (pedaling in a circle.) Just my $.02.