To add a tip to Cyco's:
Try one leg intervals. They are very good for efficient and fluent pedaling. You are not going to reach 140 rpm's with one leg, but the key is to build 'muscle memory', so your legs know what to do when they spin faster
. You are also going to recruit parts of muscle you never used before
. Once you trained them to benefit the pedaling motion, your legs have a much more constant power output throughout the pedal stoken. Your current lack of this fluent, balanced pedaling technique causes you to 'bounce', limiting your top RPM's. Think technique first, leg speed later!
If you are going to try them:
When doing one leg intervals, start with a light gear, but one that provides at least some resistance. Try to focus on technique and an even motion, not speed or force. Do not just focus on pushing down and pulling up: The most difficult motions are pushing your foot forward at the top of the pedaling stroke and backwards at the bottom. You will need them to attain a fluent motion.
Once you start to build up even a bit of lactate in your leg - stop, give the other leg a workout. Chances are that after some workouts to each leg they both are a bit tired. Just spin lightly for a few minutes to clear them up and start again. There is no point doing this with lactic acid filled legs.
Oh, and ehh... never mind the people looking at you