I have an 07 GT Convertible that I keep garaged. I try to never wash it. I have a duster and I do a quick wipe down when I take it out and then when I put it back I do a quick walk around look. If I see any spots or anything I spray some detailer or liqued wax and wipe it off. I like Meguiars products. It stays clean that way and always looks good. Also, nothing stays too long that can eat into the paint. I wash it about every 6 months when I want to clean the cloth top and spray it with protectant.
Since I keep it garaged and it is not a daily driver, it works well for me. Also, I am not driving on any dusty roads where it would get too dirty.