A computer with cadence helped me to get used to spinning, also. I've also tried the visualization of wiping mud off my shoe at the bottom of the pedal stroke. A better image for me, though, is to think of the pedal cranks and my feet as the hands of a clock-- I picture both of my feet going around in a circle just the way a second hand sweeps evenly around a clock face, and that helps to smooth my pedaling at any cadence.