Several years ago my wife and I flew to Europe from the U.S. on USAir buddy passes. Our friends had read the rules and had told us they said we needed to be dressed in something better than jeans and sneakers if we wanted to be able to buy an upgrade for $50. I thought that sounded like hogwash (I didn't understand at the time what a buddy pass was), so we showed up in jeans and sneakers. No dice, while our friends, also on buddy passes, got business class seats for $50. On the return flight, I put on my one pair of khakis and the hiking boots that were the only other pair of shoes I had brought. We almost got the $50 upgrade, until the head check-in agent saw my shoes and said, "No way am I letting you upgrade in shoes like that" (and they weren't even muddy or scuffed up or anything).

These days there's so much less left to the discretion of gate agents or flight attendants, that essentially all of my upgrades are earned. But I still dress reasonably well because it can make a difference when dealing with someone who does have discretion, or just in the way I'm treated.