What kind of cover do you have? Is it factory? if so I would
Take the cover to a laundry mat and use one of the large capacity front loader machines use a little oxy clean and wash, then place in large capacity dryer ,tumble dry (No or Low Heat) Then take it home hang outside out in the sun. Then use 303 fabric guard. http://www.303products.com/fabric-gu...uard-16oz.html
It will make your cover look and perform like new!