1) When cleaning a Mezuzah’s outer case it can be lightly dusted. But never use liquid cleaners because they may seep within the case and damage the parchment scroll.
2) As well I have seen many burnt and water-logged Mezuzos.
If your Mezuzah is going placed out doors and will be exposed to the rain and moisture, be sure to put it in a waterproof cover. A water proof cover is a solid piece of plastic with only one opening on the bottom and it is sealed airtight with a rubber cork. If it will be in contact with the sun be sure it is in a white or brightly colored case which will reflect the sun as opposed to a dark color attracting it, thereby burning or drying out your Mezuzah, which causes the letters to crack and very often renders it Possul / invalid.
As well it is important to avoid metal covers, the reason for this is two fold 1) The Das Kedoshim writes as follows; The reward for fulfilling the Mitzvah of Mezuzah is long life as the Posuk says “Leman Yirbu Yimeichem” and weapons of war are made from Barzel – Metal therefore it’s not proper to put a Mezuzah in a material that cuts life short. 2) Metal tends to absorb the temperature much more than plastic, in the winter it becomes ice-cold and in the summer it gets scalding hot,which is very damaging for a Mezuzah.
(think about it would you touch a metal railing during the winter months without gloves? or a metal railing in the heat of a summer day? I think not! )