One "mental trick" I found useful was from a post to a Road Forum thread that the force applied to the pedal is at 90 degrees to the crankarm. As I understand that, you only push directly downwards (and pull directly upwards) when the cranks are at 3 and 9 o'clock. At the 12 o'clock position you are pulling directly backwards in the mud-scraping action, and at 6 o'clock you are pushing directly forward. So the direction of applied force is continuously changing through the cycle to maintain that 90 degree relation to the crankarm. It took some conscious thought to apply the principle, but by George, I think I've got it.