Unless I missed it in all the cadence chatter here is a free way to improve your spin. Pedal with only one leg. It won't improve your cadence but you will feel the full "spin cycle". I actually unclip one foot and pedal with the other. You can also do it by just concentrating on one foot. I think this goes to the question of improving your spin and not your cadence. I've read recently that cadence is a red herring anyways.

Once you have a fluid spin I think it is easier to increase your cadence because you are more efficient.