Be courteous, smile and shake hands firmly. Your overall posture,
  gestures and facial expressions will reveal a lot about you, therefore
  come across as enthusiastic, interested and alert this will in turn create
  an overall good feeling to the prospective employer. The right attitude
  is all important to any organisation.
   
Wait to be shown where to take a seat.
   
Look the interviewer in the eye and smile when appropriate
  (a friendly disposition goes a long way, but always remain professional).
   
If you are being interviewed by more than one person, always look at and
  address the person who posed the question, then glance at the others as
  you are coming to the end of your answer.
   
Compose yourself comfortably, don't fidget, don't tap your feet or objects,
  or sprawl. Should you be offered a cigarette, do not accept. To accept a
  refreshment of some kind is acceptable if you are going to drink it
  otherwise decline as it may be an unnecessary distraction.
   
Most importantly have any mobile phones, pagers etc on silent or
  switched off before the interview.