Yeah I have this problem sometimes as I'm always hot and usually sweating while boarding. Couple things I do:
- At the bottom of the run, even in -26 deg weather I pull my goggles up onto my helmet/forehead to allow my face to "breathe"
- I wear a face mask but pull it down when my goggles are on, so that it's covering my lips and chin, but not my nose. When I pull the goggles up I'll pull the mask up to cover my nose and warm it for a few mins
- Never touch the inside of the goggles with anything, or you'll potentially screw up the anti-fog coating
- heat guns/hair dryers/hand dryers work great, but don't leave it in one spot too long...
I have a problem with moisture getting in between the lenses on one of my cheap pairs. What works for me is leaving them on the heat register in the house. Enough airflow to dry them completely but not too hot to melt anything!