You looking at me??
First impressions count!  As you walk through the door, immediate decisions are made about you.  What you wear, your hair, your make up, whether you make eye contact or smile.  Within seconds, people will have decided what their impression is - and whether that is positive or negative.
When you go for a job interview, it is important to make sure you create the image you intend. That you match the image of the position you are applying for. That you have given the right messages.
Think about what you plan to wear. I would never advise wearing something brand new because you may not feel comfortable. Maybe it is a bit tight, or the neckline is a bit lower than you thought - or the hem a bit high.  Are those shoes tight - making you uncomfortable and distracted just when you need to be thinking straight. If you have a new outfit, wear it beforehand so you can check out all those details.  Make sure your socks or tights are clean, matching and have no holes!  They may not be visible, but it is all part of the impression - if you know you have a hole in the toe, you won't feel as confident.
When you walk in through the door make eye contact with the people  you are meeting. Shake hands. Practice your handshake - you don't want to have a limp, damp one!  Ask someone you trust to give you feedback.  
If you are doing a presentation, have handouts ready for each member of the panel.  Are you giving a powerpoint presentation?  Don't look at the screen!   It's not good to just repeat everything on the slides. They can read - and will be less than impressed if you just reel off what is there. Use the slides as a prompt to tell your story.  Be passionate and make them believe in your passion.
Stick to time!  It is disastrous to go over the time limit. The worst that can happen is they cut you off, before you have given the story you planned.  Or they can let you drift on, but mark you down for poor planning and timekeeping.
Practice in front of someone (or a mirror) and time yourself so you know you have the timing worked out.
Answer the question!  You would be surprised how many people don't . And if you are unsure of something, be honest - don't flannel. You know they will spot it.
There's lots more...interviews are daunting and you need to be prepared. Seek out some help to make sure you get it right.