Tricks for keeping glass from fogging up racing on cold nights?

Living in SoCal I never have had problems with the windshield fogging while racing at night.
However, one time years ago at OCIR I was wearing a full-face helmet with a shield during nighttime bracket eliminations. Two or three cars back from my next run I brought my hands up to the bottom of my helmet and nervously blew on my hands to warm them up. That immediately fogged up the inside of my helmet face shield!
Luckily it dissipated before I needed to stage.
Sure learned my lesson.