Practice, practice, practice (assuming your seat isn't too high, which would also make your hips rock from side to side besides bouncing). Ride rollers when the weather's bad. Do downhill intervals - climb the hill, then ride down it in the same gear you climbed in, spinning as fast as you can to barely keep tension on the chain. Visualize your feet pedaling in circles, not squares. Do cadence intervals on the road - ride one minute at 90 rpm, then rest, then one minute at 100 rpm, then one minute at 110, and keep building until you can't maintain the minute without bouncing ... then go back down the ladder again.

There isn't necessarily one cadence that's the most efficient for everyone, but you ought to be able to spin with good form at any cadence. High RPM work helps build that form, because it unmasks the flaws in your pedal stroke.

And make sure your crankarms aren't too long for you.