Last year was my first year cycling through the winter and at very low temperatures I constantly struggled with problem of fogging glasses. This year I'm using a combination of a neoprene mask and a half-mask respirator and so far I've been able to ride at -32C with zero fogging. On the first test runs, on the coldest days, I experienced a slight fogging of my glasses inside my goggles but the dishwashing soap trick has taken care of that. It is such a relief to not have to worry about losing visibility during my commute.
I have posted more details here if anyone is interested.