All cheap solutions starting from ripping foam, baby soap and antifogging are working only to the certain degree. Thermal lens may help, buy if you are big, fat, sweating guy like me, it is simply not enough. Invest into Turbo Fan goggles, and do not use helmet to have head cool. In my case I installed separate set of fans into helmet.
Think about it as investment to expensive optics to your gun. This is literally last place you want to save money.