Be honest.
Don’t be cocky, but don’t undersell yourself. Interviews are not the time for modesty.
Do your homework. Know a bit about the company, where they are based, and what they do. I had one candidate, on a phone interview, tell me how he really wanted the job, because he wanted to move to London. He had visited once before, and loved it, and really wanted to build his life there because it is such a wonderful city. However, we are actually based in Glasgow.
Think about the typical interview questions you might be asked, and how you might answer them. There’s a common set of questions that often get asked (“So what attracted you to this post”, “How would your colleagues describe you”, “tell me about an occasion when things were going wrong, and you had to recover the situation”, “Tell me why I should employ you rather than the person that I interviewed half an hour ago?")
When you’re asked if you have any questions, ask something sensible, positive, and not something that you should already know, or was answered in the interview pack. It’s better to ask “So is this a new position, or did someone leave?.... why did they leave, was it an internal promotion or an external move?...” or “What training would you expect me to have carried out within the first few months” rather than something like “How long are the tea breaks” or “What’s the pay?”
Make eye contact with whoever is asking the questions.
Remember, you’re being assessed from the moment you arrive. The interviewers may ask anyone that has spoken to you - receptionist, security - for an informal opinion. I have once been on an interview panel where we stopped for a break between candidates, and the janitor said “that last candidate was a bit strange, and very rude...."
Relax.
Do not be an arse.
Do not fall off the chair. (yes, that did happen to one person I interviewed)
And remember, the interviewers are not out to “get you” or to ask you trick questions. They genuinely want to get the best person for the job, and a good interviewer will help you to give the best answers to demonstrate that you are the best candidate.