Goggle lenses are treated with a coating on the inner and outer side to optimise vision and reduce fogging. This finish can be quite delicate particularly on the inner side, avoid using chemicals in cleaning agents such as “Windex” type cleaners as although they will clean your lens the ammonia will eventually turn the lens brittle and cracks will begin to appear and run through the lens.
The best method is light soap and warm water. Submerge the goggles completely and gently work the soapy water through the goggle foam, if there is any mud built up, allow the water to soak the mud until it falls off the lens, you can further clean the goggle lens with a microfiber cloth or using your goggle bag.
You should also wash your goggle bag regularly as you can use this at the track to clean your goggles on race day. Do not use a tee shirt, towel or your motocross gloves to wipe dirt off a lens as you will grind dirt particles into the lens which will scratch them.