Dress business casual. It's a subtle but significant difference. Keep in mind what must be going on in the mind of UA employees.

I know that they'd like to think that they treat everyone the same, but here's two things that shape the discrimination that UA employees use everyday with just about every passenger they encounter:

1. Because of the marketing systems that divide customers into status groups, the overall mentality of any line employees is that some passengers more worthy of help and generous service than others. That basic mentality of legal discrimination is why I think that anything you can do to conduct yourself within the profile of what appears to be top tier will give you a slight edge in the long run with any matter in which UA employees have discretionary powers.

2. UA employees have a dress code that they must adhere to when flying standby on UA. I think that this has a subtle affect on their mental picture of who deserves special treatment and who should be ignored or refused.

I almost always fly with tie and jacket. I have no way to prove it of course. It could be that I'm superstitious, but I know that I've been given extra consideration in a lot of small ways. It's not always upgrades. It's things like extra checking to explore options to protect me on other airlines when I've misconnected because of weather. Or responding to a kind request on board to adjust the temperature on a freezing airplane. Or the baggage claim people offering to have the late bags delivered even though the check-in agent wrote a comment that I had checked in late. Or an LH agent giving me my choice if I wanted to use an SWU or a miles voucher for my upgrade (when strict policy required her to collect the voucher).

May I stress that the most important thing to put on is an attitude of kindness, cheer and respect. The easiest way to have the worst possible journey is to approach airline employees with an aggressive, demanding attitude of entitled supremacy.