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.
If using a commercial glass or mirror cleaner, be sure to use a standard brand of glass or mirror cleaner that does not contain ammonia or vinegar. Do not use Windex.
Never spray any cleaner directly on to a mirror. Instead, apply the cleaner to a soft cloth and 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 that no commercial cleaner comes in contact with the edges and backing.