I just put my '05 GT away for the winter and finished the process off by covering her with a car cover called Dustop from California Car Covers. It fit perfectly and the quality is very good for the price. I highly recommend it for anyone storing their Mustang indoors. Here are pics and more info:
http://franksperformance.com/Forums/index.php?showtopic=237&pid=737&st=0&#entry737
http://franksperformance.com/Forums/index.php?showtopic=237&pid=737&st=0&#entry737