I came across this link on another site, and it seems it would be useful to folks here. It concerns how to keep one`s mask fog-free.
The quick summary - though you would do well to check the link:
1) Outside water is colder than inside air. That`s why, as with a cold drink on a summer day, you get condensation.
2) On a prepared mask, water will condense as a sheet of water, not droplets, so it won`t interfere with your vision.
3) Using a toothbrush, scrub the inside of the mask with non-gel toothpaste or similar mild abrasive.
4) Rinse in warm water
5) Chill lens
6) With mouth exhale deep, steamy breaths onto lens. It should not fog.
7) Anti-fog solutions, including spit or baby shampoo, reduce surface tension, allowing condensation to spread smoothly and invisibly. Rinse the mask, then apply defogging solution with a CLEAN fingertip.
8) Keep greasy products away from mask. Oil + water = fog.