Glasses fogging trick - tape the nose bridge area of the mask. Been doing this for the 3M dust masks as long as I've been riding fix gears and that a time span of eras. I always used masking tape (because boat builders always have an infini8te supply). I take the new mask and put a 3" strip of say 1" tape along the top edge, centered at the nose, then another piece just below. Looks odd but works. Never tried it for cycling but I don't see why that would be so different.

Edit: I have always used 3M dust and asbestos masks, the ones with two yellow straps. Now my last box (from Home Depot) has masks that are smaller and I don't get a goof fit so I am going to look harder next time. But sort of that, those masks are good. They hold up and can be washed. Straps fail first. I could never get an acceptable fit with any single strap mask. My two strap 3Ms stopped fiberglass dust at two weeks old, working 40 your weeks than a single strap mask brand new.

Ben