Three ways to prevent your glasses fogging up while wearing face masks:
1) Wash your glasses in soapy water, do not rinse, then allow them to air dry. This will cause a thin film of surfactant to form on the glass and repel the vapor from your breath. It will last a few hours 1/3

2) An absorbent or impermeable barrier between the top of the mask & your glasses. Some masks are made with a built-in foam strip at the top. You can replicate this by placing a folded-up tissue tucked into the top of your mask to catch the water vapor before it gets out. 2/3

3) There are also technologies that prevent water vapor from condensing on glass. It is the same concept as the soap but in longer-lasting form. However please be careful of any industrial, automotive, etc. products as they may give off fumes that are not safe near your eyes. 3/3