• The best and safest cleaner for a mirror is clean, warm water used with a soft cloth. Be careful not to allow the edges of the mirror to get or remain wet over a period of time.
• Use a standard brand of glass or mirror cleaner that does not contain ammonia or vinegar.
• Remove surface marks or stubborn dirt with oil-free steel wool, not solvents. These could attack and damage the edges and backing.
• Never spray any cleaner directly on to a mirror. Instead apply the cleaner to a soft cloth, then wipe the mirror.
• Always use soft, grit-free cloths when cleaning a mirror to reduce chances of scratching the surface.
• Be sure to dry all joints and edges thoroughly to be certain no commercial cleaner comes in contact with the edge and backing.

-- Tips courtesy of HomeAdvisor.com