Do your research
I am sure you have heard it a thousand times but, fail to plan and you plan to fail. You are certain to be asked specific questions about the company, so make sure you've done your homework on things like last year's profits and latest product launches. Also, take a look at the latest developments in the industry so you can converse with confidence. If possible familiarise yourself with the interviewer, LinkedIn is a great source of information on specific individuals and gives you a good idea of the people within the business, where they worked previously, their job roles and where they were educated.
Practice your answers.
Although there is no set format that every job interview will follow, there are some questions that you can almost guarantee will crop up. You should prepare answers to some of the questions you will be asked regarding your personal strengths and weaknesses, as well as being able to explain why you would be the best person for the job. Make sure you know your CV, too many interviewees appear to have only a vague knowledge of their career history, it is amazing how often this comes up.
Look the part.
This is really basic, but it is worth reiterating because it will have an impact if you get this wrong. Appearance's matter, you are often judged before you've even uttered a word. Make sure your shoes are polished, your clothes fit correctly and that your accessories are subtle. Ensure you have a pen and pad, it is incredibly basic, but people do forget these.
Good preparation is the key to staying in control. Plan your route, allowing extra time for any unexpected delays, and get everything you need to take with you the night before such as extra copies of your CV, a pen, folder (not a cheap pen and a scrap of paper), your portfolio (if applicable) and any supporting documents to enhance your application. Remember to speak clearly, smile and remember that your interviewers are just normal people, they may be nervous too!
You should always have some questions for your interviewer to demonstrate your interest in the position. Prepare a minimum of five questions, some which will give you more information about the job, and some which delve deeper into the culture and goals of the company.