It's easy to tell who they are. In any group ride just look around. They'll be the ones pedalling with a motionless upper body and legs spinning effortlessly. They don't waste energy with swaying, mashing or square pedaling. They are elegant and economical. How did these economical riders get that way? Part of it may well be genetic. They were the lucky ones whose parents gave them a neuromuscular system meant to pedal a bike without effort. But there is also a nurturing or learning-and-perfecting component. This is the best part.
We can all learn to become more efficient at pedaling. By doing so you'll have more power in reserve at the finish for the sprint, you'll climb without trashing your legs and you'll have more physiological "gears" when the pace changes suddenly. At the core of this learning portion is the ability to spin comfortably at high cadences. If you learn to pedal at 120 rpm without any stress or strain then it's nearly certain that you're more efficient. So how do you develop a higher cadence? It all starts with understanding pedaling basics.
The most basic skill in pedaling with a high, efficient stroke is learning to pedal horizontally. I'll explain. For this purpose, let's think about the pedal stroke as having four sides--the downstroke (2-4 o'clock), the bottom (5-7 o'clock), the upstroke (8-10 o'clock) and the top (11-1 o'clock). The least critical of these for becoming more efficient is the downstroke. Anyone can do this. Put a cadaver on a bike and it could also make this movement. To become more efficient you need to get good at the top of the stroke. This is the hard one. Try pedaling with only one foot clipped in and this will soon become apparent. Drills which emphasize this region are what you want to work on.
The best drill is the one already mentioned--one-legged pedaling. This best done on an indoor trainer. One of the best teaching tools for this, although an expensive one, is the Power Crank. I'd recommend putting these on a second bike set up just like your racing bike to save the hassle of putting them on and off frequently. One legged pedaling will produce good results if done two or more times a week for several weeks. More weekly sessions will shorten the skill development time. This emphasis on becoming more skilled at the top of the stroke is why I suggest to the athletes I coach who need to become more efficient that they mount Q Rings on their bikes and ride them all the time. They are not a teaching tool. You use them not to learn to become a more efficient pedaler, but rather to pedal more efficiently--right from the start of their use. (And to set the record straight, I am not paid anything to promote Q-Rings or power cranks.)
Improving pedaling economy is one sure way for a rider to become more competitive. I have the athletes I coach work on this weekly throughout the Base period with periodic "refreshers" during the rest of the season. One of them has gone from a time trial cadence in the low-80s to the high 90s. But he needs to refresh his skill frequently to maintain this ability which isn't natural for him.