if you are hoping for an upgrade, you need to dress well, so that the agent might put those magic letters SFU (Suitable For Upgrade) in your file.
Oh yes, I forgot to mention that most gratifying feature of going for smartness rather than sloppiness on long haul flights! When I was still working, I was usually booked on business class by my firm, and I generally asked them to put me on BA flights where possible, so the trips were never uncomfortable. But with my quaint old-fashioned custom of never dressing down, I was several times upgraded without asking, to first. And some friends in Guernsey, who are frequent travellers on "SKI" exotic holidays ("Spending the Kid's Inheritance"), share my sober dress sense, so very often find themselves with free upgrades! On their last trip, the couple of friends accompanying them were dressed entirely for comfort rather than style, and suffered the humiliation of being rejected when they asked to be treated similarly.