Oh, fer Chrissakes, this is not difficult, nor is it a Black Art. Be clean, be smart. Wear a suit and tie. If you haven't got one, go and get one - as Pete says, do it with some feminine input so you don't make too much of a howler. Do NOT attempt a fashion statement, and do NOT attempt a pre-emptive strike for dressing-down in the workplace. Wear clean, polished, sober shoes. I don't care if they're loafers. I also don't care if your tie has red in it, but don't distract me with a cartoon strip on your tie - I want to read you, not your clothes. In fact, that is the whole point. Your clothes are not the centre of attention here; you are. So don't rock the boat. Let your brain do the talking via your mouth!
The same applies generally to the ladies. Pants or skirt (not too short) isn't really important, but the overall image should be sober and smart.
Also, don't put too much store on the comments of those with only one or two interviews behind them. Again, as WWW said, an interview preparation book may be a good investment.