Remember that your interview starts from the moment you walk into the building until you walk out. (One company I worked for always asked the receptionist also for feedback!)
After they have introduced themselves - try to use their name once during the interview.
Back up any negatives with positives eg In the what are weaknesses question, demonstrate how your weakness can be a positive too.
If you don't understand what question they are asking - don't be afraid to ask them to clarify it further. This can also buy you time to formulate the answer in your head.
If you answer a question and feel that you are unsure about the answer - ask them - 'does that answer your question?'
Ask them what kind of person they are looking for in the job, and be sure to use examples of how you fit those requirements
Research the role as well as the company and then use the interview to say why you think your skills would be an excellent fit for the job.
Research any special terminology that in your career area and also the business area the company is in. If you can - try and use these terms too - but only if you feel comfortable using them.
Try to relax and remember that they are asking you about the easiest subject for you - yourself!