My Jag lives outdoors 24/7/365. After shopping around, searching the Autopia threads, and PMing several Autopians, I got the Supremeweave from California Car Cover. I needed something tough enough to withstand our regular hail storms, as well as affording UV protection.
As SVTHorsnake mentioned, you also must take some precautions in your use of the cover. Only apply it to a truly clean car. Never "drag" it along the paint. (That's gonna pose problems if you have to install/remove it by yourself.) Search on "car + cover" and you'll find (as I did) even more practical, helpful tips than we can offer in this one thread. :up