Training at a specific power level is considered to be a better choice than training at a specific heart rate.

Cadence is really orthogonal to that. Having a higher cadence range is useful and spinning more can help, assuming you have enough aerobic capacity to deal with the higher cadence (the theory is that spinning faster shifts the burden from the muscles onto the aerobic system). I think this works, but it's not a huge effect, and for it to work, you need to not only be able to ride at a higher cadence, you have to do it efficiently.

To increase your cadence range, try the following intervals:

1) Pick a gear in which you are riding easily, and slowly ramp up the cadence to where you start bouncing in the saddle. Back off a bit to a point where you are spinning easily *and* your heart rate isn't skyrocketing. Concentrate on spinning lightly and pedalling in circles - pushing over the front and pulling back across the bottom.

2) Do a recovery period as long as the high-cadence interval.

Repeat this 4 times. Initially, I'd suggest that you do an interval of about 1 minute, and add 30 minutes every few times until you're up to about 3 minutes.