My indoor cycling class (my bike on their stationary trainer) includes several drills to smooth pedal stroke and increase sustained cadence that includes many of the suggestions already posted. Single pedal drills: 30 seconds on one leg, switch and 30 seconds on other leg, repeat total of 5 times. You'll know you're getting tired when you feel a dead spot near the top of the stroke.
Spin ups - somewhere between 30-60 seconds at a fast cadence, then increase by 5 rpm and hold, increase by 5 rpm and hold,... then step back down. Recover for several minutes (5), repeat. The instructor suggested we could work on this even just 1 time per outside riding between classes and see benefits; which I've been doing on my commutes. Ask me in a few weeks if it's working (although I think it is, really too early to tell).