I can't help you because I have the same problem!! I bought a tan colored one a couple of years ago and that one got mildew stains on it and turned an ugly color. So last year I bought a new one in black, that was supposed to be outdoor non fade fabric, it was twice as expensive as the tan one and you should see it this year!! Yuck!
It looks just as bad, it is faded and ratty! I guess this is something that I will just have to keep investing in every year!! I was thinking that I might make my own cover using some oil cloth which is a water proof fabric used for table cloths etc....