Beware of Helen's advice. Someone fiddling with a pen continually is VERY obviously nervous. And doing it below the desk could look suspicious, depending on how you're sitting... ;-)

My tip is to wear quite a heavy, loose-fitting top. If you're nervous then you sweat, and enormous damp patches under your arms aren't attractive. I've got some heavy-duty Oxford shirts which are great for interviews - the sweat can be rolling down your armpits to your waist, but the material's so thick, no-one can tell!

Graham.