Although you don't say what kind of supervisory position it is (front line, middle management, etc.), I agree with the others that you can never go wrong by wearing a coat and tie. I don't think it needs to be a suit, but a nicely coordinated sport coat, trousers and shined shoes will do it.
I used to do the interviewing for our Transit department. The positions people applied for were part time drivers and maintenance department positions. I did not expect anyone to walk in wearing a coat and tie. But when people come in wearing bib overalls with no t-shirt, or any shirt for that matter, I drew the line there.
As Jim mentioned, wearing nice clothes shows respect for the potential employer. But it also shows that you have respect for yourself, and that's important too!
I once attended a workshop about proper interview techniques and procedures. One thing that always stuck with me was when the instructor commented that the interviewer should always look at the back of the applicants shoes as (s)he walks out. If the back of the shoes are shined as well as the toe of the shoe, that is a person who is thorough and well disciplned. If the back is not shined but the toe is, that's indicative of a person who's willing to take shortcuts and probably doesn't have a lot of self esteem.
For what it's worth, based on my experience with the people that were ultimately hired, I'd have to say I agree with that little tip.
So shine the backs of your shoes! /forums/images/graemlins/wink.gif /forums/images/graemlins/grin.gif
Good luck on your job search. /forums/images/graemlins/smile.gif