My scuba instructor taught me that when you get a new mask, there will be a very thin layer of greasy stuff on the lens which is left over from the lens manufacturing process. What you need to do is to use toothpaste, not gel, to remove this layer. Make sure you wash it out thoroughly, otherwise you will return with burning eyes. Having done that, I use defogging spray before the dive and then rinse it with sea water before decending. Spit in the mask also works well.