I think you are on the right track. Thinking about circles or about wiping mud off the bottom of the shoes or whatever mental trick you use, it's all about developing a smooth spin and transferring power as evenly as possible through the full circle of pedaling. The longer you do it, the more natural it will become. But even after many years of pedaling circles, I still find a boost in power by concentrating on smooth pedaling at key times like while climbing or when accelerating on flats.