I use disposable surgical masks, the type with elastic loops to go behind the ears. Pinch the metal strip over the nose and it directs breath away from glasses. My safety glasses are fog resistant already. They worked for me down into the low 20s (F) last winter. Keeps my breath warm and humid so I don't get asthma attacks and respiratory irritation.

Hygienic, convenient and cheap. If you don't care for the common blue colored masks you can order them in other solid colors and patterns.