I'm working with a young lady who is just getting into cycling, in preparation for doing triathlons. We were out two weeks ago, working pretty hard on one section of our ride. I pulled up next to her and said exactly that..."Wiggle your toes"...then "Relax your shoulders". You could almost see the tension drain from her face.

Personally, I'm a big toe clencher and have to remind myself all the time to relax my feet.