I must admit, when on a full fare f or j ticket (where there is no F class available) I tend to dress in jeans and a shirt and "God forbid" flip flops - I am fairly young compared to many other passengers, and if I were smart, folk would wrongly assume I had been upgraded and worked for the airline. I tend to look at other smartly dressed youngsters and assume they are staff! Having spoken to other passengers in this catorgory, this is usually the case! So for me casual every time!