Switching to clipless pedals will enable you to spin your pedals in a circular motion, but for a leisure cyclist, its a big investment. A $10 set of toe clips will enable to apply power over more of the pedalling circle. They are not quite as efficient as clipless, but are a quantum leap compared to plain platform pedals. For safety, and ease of release, never tighten the straps.

Things to practice incluse:
Not stretching the cranks. When you reach the 6:00 position, dont try to press down further.
On the up stroke, relax your legs and switch from a push to a pull as swiftly as you can. Most riders still apply some downward pressure on the upstroke.

Pedalling in circle is something to practice in a low gear, at a slow riding speed. Dont try to combine riding fast with pedalling fast, until you have a bit of practice.