I use a drop of dish soap and gently rub it on then kind of wipe it off. Works wonders for me when I'm heavily sweating, nearly no fog and virtually free.
I SCUBA, and spit always worked best when diving 😄
Same here. Might not be the best solution for a HMD though. While I had DK2 for a year I always used to have a microfibre cloth available and was reducing amount of beer consumed while playing.