Sixty Fiver - Not disagreeing with you here, a fixed gear will definitely force you to spin on the descents. However, it takes a lot of discipline to ensure a full pedal stroke instead of one aided by the forward motion of the drivetrain. I know one thing that has helped me is to run a slightly looser chain (on a fixed gear) so that I can feel it when I'm slacking.

That being said, it also takes a lot of discipline on a geared bike because at any point you can just let go and coast.

Either way, you have to really want it.

Also, I think there's a difference between learning how to spin quickly and smoothly, and learning how to pedal efficiently. I know that back when I was still using clips and straps, I was able to spin 150rpm smoothly, but I wasn't actually powering through the entire crank rotation. When I switched over to clipless, I paid a lot more attention to powering through the rotation evenly, which has helped both my form and, more importantly, my efficiency on the bike. I think when people begin focusing on high rpm spinning, they assume that as long as they are sitting still on the saddle and maintaining smooth form, they've mastered it. But how the power is applied through the crank rotation has had a greater effect on my cycling, personally.