Well it kinda depends on your helmet. If the lenses fog up because of your breath then you could put some foam under the eyes so it blocks the air from going up. That's what I did with my Chaos Marine helmet and it works perfectly. But if there's not much space in there then the lenses are going to fog up no matter what you do, especially when you start sweating. There's also anti-fog spray that you can put on the lenses but I have no idea if that actually works.