I, too, only "wash" my GT and other cars (and I've owned about 10 black cars) when I get caught in some rain and cannot avoid heavy dirt caked on the paint. I keep a great coat of wax on them (polish a couple times a year prior to wax) and use a duster whenever I can. Next in line is the car detailing spray and micro fiber towels. When it comes time to really clean, I take a wet 100% terry cloth towel and gently wipe down a section of the car, then use a 100% micro fiber towel to dry. Rubbing and drying paint with terry cloth towels scratches paint surfaces, micro fiber towels do not. I even use this method on my 65 Mustang coupe that has two year old Acrylic Enamel Raven Black paint without clear coat, just like when it came off the factory line.
I received a flannel car cover last week from California Car Covers. Fits superb, as does the Ford cover that came with the car, but that's very light weight, nylon I think, and I believe will scratch the paint.
Great hearing all the different methods of how we wash our cars, and they all come out great looking.