An interesting advice from Nikon support – do not breathe on your lens to clean it because you could damage the lens coating:
How do I clean the camera lens?
The best way to clean a lens is to use a piece of lint free lens cleaning tissue and a small amount of Lens Cleaning solution. Do not use anything containing abrasives or solvents, only use Lens Cleaning Solution.
First we recommend taking a small blower brush to blow off or brush away loose dust or debris.
Next, place a drop or two of cleaner on the tissue (never directly onto the lens) and then wipe the lens in a circular motion, beginning in the center and working your way outward, removing any marks or smear.
If the above supplies are not available a clean, dry, soft, lint free cloth can be used to clean the lens. Do not breathe on the lens to fog it for cleaning. There are harmful acids in breath that can damage lens coatings. Just use the blower bulb, then brush, and wipe the lens in a circular spiral from the center outward.
The same method can be used to clean the viewfinder eyepiece of Nikon cameras.
