Actually you can spin up a hill - make the upstroke of the pedal on the back count as much or more than the downstroke (the mash) on the front...

A guy I know told me to practice spinning by pedaling with one foot unlipped and off the pedal - meaning only one foot and leg doing ANY work - the other hanging in mid-air out to the side. Do this on both sides until you are feeling the full rotation of the pedal, not just the mash or downstroke at the front of the pedal.

Then spin with both feet! AMAZING how much easier the hills are!!!!!! I can go up them almost twice as fast as I could!