Travel to your interview in good time. Take a walk before you go in to relax. Turn up modestly early, be polite to reception staff and chatty with other interviewees. People do notice these things and the internal grapevine can be very efficient. Walk in confidently, smile, and offer a firm, but not crushing handshake.
Body language is critical.
Sit up straight, hands in front of you. Don t fiddle. Eye contact matters more than anything, though it should be light, not startling. In panel interviews don t just look at one person - you want them all to form a favourable opinion of you. An interview is a dialogue, not a monologue so don t give one word answers.