The answer is repetition with variation. As with any motor skill, pedaling becomes naturally more efficient as we practice the movement over and over. The more often you turn the cranks of your bike, the more your pedaling efficiency will improve (as long as you don't exceed your capacity to recover from the fatigue that results from high-volume training).
Simple repetition alone is not sufficient to optimize the rate of improvement in pedaling efficiency, however. To get the greatest benefit from each turn of the cranks, it is important to constantly vary your pedaling in a number of ways.
By variation I mean, specifically, challenging the limits of your pedaling capacity in different ways. Throwing disparate types of pedaling challenges at your neuromuscular system forces it to get creative--to try out different patterns of muscle recruitment, some of which will be more efficient, others of which will help you resist fatigue better.
Each bike workout you do within your weekly training regimen should be somewhat unlike the others. Following are the five major variables you should manipulate in order to challenge your neuromuscular system and stimulate rapid pruning of waste from your pedal stroke.
Speed/Intensity: The most important variable to manipulate in training is speed, or intensity, because fatigue results from different causes at different pedaling intensities, and experiencing fatigue from different causes stimulates performance-boosting physiological adaptations.
The four major speed/intensity levels you need to incorporate regularly into your training, in order of decreasing volume, are moderate aerobic intensity (a comfortable but not dawdling pace), threshold intensity (roughly 40km race pace), VO2 max pace (a pace you can sustain no longer than 10 minutes) and maximum power.
Duration: The greatest stimulus for fitness adaptations occurs when you pedal in a fatigued state. This is when your neuromuscular system really has to get creative to find new patterns of muscle recruitment to sustain a desired speed. Obviously, fatigue develops at different rates at different intensity levels, so you will need to manipulate the intensity and duration workout variables together.
You don't have to ride to the point of extreme fatigue in every workout, but you should complete one long-duration/moderate-intensity ride ending in moderate to significant fatigue and at least one high-intensity/shorter-duration ride ending in moderate to significant fatigue each week, most weeks.
Gradient: Pedaling uphill presents a very different physiological challenge than pedaling on level terrain--that's why new leaders emerge when the Tour de France hits the mountains each year.
You should experience challenging hill climbs in at least one bike workout per week (either in the form of hilly longer rides or in the form of high-intensity repeated hill climbs). But not all of your rides should be hilly--otherwise, where would the variation be?
Force: Pedaling in high gears presents a different sort of challenge to the neuromuscular system than pedaling in lower gears. Your muscles must produce more force to sustain a given speed in higher gears.
This is also the case when climbing hills, but equivalent levels of force application represent different physiological challenges on hills and flats due to the differences in joint angles and gravitational resistance in the two situations. So it's beneficial to challenge your force production capacity by cranking up the gear ratio and pedaling hard sometimes in training.
Cadence: You can produce a given level of power (speed) at various pedaling cadences, only one of which will feel most natural; all others will feel either slower or faster than natural. Pedaling at faster and slower than natural pedaling cadences also challenges your neuromuscular system to adapt by finding new efficiencies.
Pedaling in high gears automatically constitutes a slower-than-natural cadence training challenge. A good way to challenge your capacity to produce high cadences is the spinout drill, where you shift into your lowest gear and pedal as fast as possible for 30 to 60 seconds.
A small amount of formal cadence training is sufficient. But I also recommend being a little playful in your gear selection and pedaling cadences in most of your normal training rides, sometimes resisting the urge to shift into a lower gear and increase you cadence, other times doing the opposite. This will add another small layer of variation to workouts whose primary purpose is other than formal cadence training.
Active Expert Matt Fitzgerald is the author of several books on triathlon and running, including Runner's World Performance Nutrition for Runners (Rodale, 2005) and his newest, Brain Training for Runners.