My SA has been really good about this, has his own laminated "Don't Wash" sign he puts on the dash and parks the car far, far away from the gen pop. Last time I was in, I had my own Don't Wash - Thanks! sign printed in giant font (not laminated) and threw it on the dash when giving the car to the valet. They took it with them at the end, but I've got add'l copies in the trunk compartment for use any time I drop it unexpectedly!
Easy insurance - I've had the exact problem as you before.