Look very smart
firm handshake but not bone crunching
smile but not madly
research the company and job thoroughly
think of answers to questions you'd ask yourself (look at your own cv and see what q's it begs - i.e. gaps/short jobs etc)
Have some questions ready to ask them, make sure you ask at least 2
listen to what they're telling you and answer accordingly - i.e. if they say 'we're looking for someone with experience of abc' when you get to talk about yourself you can say 'my main experience of abc was at xyz company when I did fgh'
listen and don't interrupt the interviewer
you don't have to answer immediately, it's ok to say I just need to think about that for a second
Be honest
Be yourself - no point in selling yourself int a job you can't do/where you won't fit in
don't mention children if you can help it, other than in an empathising with the interviewer kind of way if it comes up
Don't ask too many 'what's in it for me' questions, i.e. 'what time do you finish every night?' - you've got to show that you're interested in the JOB, not convenience/hours etc (even if it is the latter)
Reiterate that you're interested at the end (if you are)
Don't say 'thank you for your time' - your time is valuable too. And don't say 'see you again' - you may not, ever! You can say 'it was good to meet you'
Keep your right hand free at the beginning and end for handshaking and follow the interviewers lead on when final one happens. Usually in the room ime and before showing you out
Try to show you've done your research by asking a q that proves it - i.e. 'I read that you are thinking of buying xvs company, what would be the impact were that to happen?'
Think of examples where you can demonstrate the competencies they're looking for so if they're looking for someone who has experience of customer service they may ask something like 'give me an example of a situation where the inital result wasn't what you wanted but you managed to resolve the issue' You can think if some examples before using the STAR technique: the Situation, the Task at hand, the Action you took and the Result.
And remember, they WANT to find a good person who can do the job. They should be trying to establish whether you've got the right experience - your job is to convince them you have. Good luck! HTH.