I store mine outside and only cover it during the winter for about three months.
When its beyond dirty i usually wash it with Meguiar's wash and wax.
https://www.google.com/search?ei=vsI....0.ryn2SkRJpz8
i wax it twice a year once during the summer two week vacation trip and again during my christmas/new years vacation trip with Meguiers carnauba wax
https://www.google.com/search?ei=rMM....0.HbvbOOHux4s
This year i also washed the roof and put 303 on it.
Its alot of work but it still looks like new.