I've gotten lazy and turned into a masher. Sometimes it's OK to mash, but on a tough hill you want to be able to switch back and forth between both.
But you can build up your cadence by dropping a gear or two and focusing on rapidly spinning circles--pushing and pulling at the same time.
Try riding up a hill that you can't but spin up.
Of course mtn biking up some loose gravel will teach you to sit and spin.
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