NEVER spray any weird anti fog coating on a thermal lense unless it has been specifically designed for thermal lenses! The only reason a thermal lense will fog is when the anti fog coating has been scratched off (using paper towels on the inside does this very quickly) or there is a lot of moisture between you and the lense. So you need to eliminate this problem, sweat is the big causer here, so wear a headband if you dont already to stop it.
Believe it or not, the first reply you got was farily sensible, breathing. Breathing rapidly through your mouth will fog up your lense quickly, while breathing slowly through your nose will keep it clear.
This is all experience Ive had with my I3's so hope it helps