I never dress for flights. I almost always where what I wear for work, jeans or cargo pants and a polo shirt or casual button-down shirt. Occassionaly a flannel shirt in winter. I dont think it matters. I have read in European magazines that it matters a lot on some European airlines but not at all on American ones. The reason being that the upgrades are strictly based on a status/fare formula and kicked out by the computer before the GA even sees you. Thats not to say there isnt the odd exception here and there but it is VERY rare.

People are just not that aware of attire (within a reasonable range) anymore. I see people wearing everything from jeans to white tie at the opera for instance. Ive worn everthing from dockers to black tie to the opera myself. It just doesnt have the association it did even a few years ago.

On the other hand I am very polite and respectful of people and typically make a point of saying a polite hello, or good morning or such before asking them to check a seat or what ever. As people said above, this may help and I think it is likely to help a lot more than how you dress. This is true even if I am having a problem caused by the airline and am upset. I dont take it out on the employee. As in one time I got bumped from an upstairs aisle to a downstairs middle seat on an NRT-JFK 744 a few years ago. I showed the information with the old seat assignment and dates it was made etc to the CSR and politely said I know that it wasnt her doing but I was very disappointed this would happen to a 1K flyer and would appreciate anything she could do to help resolve this. She moved me first to a window as it was available, then a downstairs aisle, then op-ugraded me to first as I was boarding. I was wearing jeans but I was being polite and respectful. I think that is a lot more important.