rain-x anti fog, you buy it at halfords for the insides of car windows, will last an eternity and stops my save phace, revision saw flys, bullet ants and other airsoft glasses from steaming up
thermal lense is the best, but using a decent antifog spray is good, and the cheapest option is to spit on the inside of the lense, rub it around and wipe it out!
Also, when you breath out, try not to like dump your breath heavily with a wide mouth. This causes alot of condensation in your mask and fogging. Basically just control your breathing, grab a thermal and all is fine
If you are using proper thermal lenses like Dye uses then don't touch the inside of the lense or spray it with anything. There is a coating on the inside and if you trash that you will have problems.;