Before the interview
Being prepared for your interview will give you the confidence to perform well.
You should spend some time researching the company you may be working for. How long have they been established for? Who are their competitors? What makes the company different?
Try to find out what the day-to-day dress code is. If the company has a relaxed dress code then you may be able to get away with a more smart-casual look for your interview.
You will likely be given an opportunity to ask some questions of your own, so plan ahead the things you might like to ask - for example things that fall outside of your actual duties such as opportunities for progression or company culture.
You should also be prepared for questions that you may be asked, think about why you are a better fit for the role compared to other applicants. Refer to the job description and identify the attributes, skills and experience you have that complement the job.
During the interview
An interviewer is looking for someone with confidence. In the interview be aware of your body language. Give a firm handshake, sit upright and smile - but don't over-do it.
If you don't know the answer to a question be honest - any floundering will be noticed - try saying something like 'I don't know the answer, but I know where to go to find out'.
Do not be negative about your previous employment or colleagues.
After the interview
Following the interview you will naturally want to find out how you did as soon as possible. Keep in mind that there may be other interviewees who have yet to be interviewed and you may not hear back until these have taken place.
Consider asking when you might hear back in your interview. Armed with this knowledge you will know when the right time to follow-up is - and don't forget to remember who interviewed you so you know who to contact.
If you do not hear back after following up, leave it a short while before trying again. You do not want to appear desperate.
Your job search does not end with an interview, even if you feel it went well. You should continue to search for jobs. Something even better might come along.