You have probably gone through school being told all of these different interview techniques (wear a suit, answer question x with answer y etc)
Ignore that. Well the suit is ok but it depends entirely on the job, otherwise a shirt and decent trousers will do.
The best thing you can do is just be yourself. Try to engage the interviewer in conversation, not about the interview but about anything. It makes it a lot more comfortable and they get to see the real you, not the fake you that has clearly been practicing your answers in your head.
For example I noticed in the last interview I had the guy had an injured hand, so I asked about it. It turns out he was in a motorcycle accident, cue 10 minutes of motorcycle related discussion.
Obviously you still need to put some effort into answering the interviewers questions and convince them you are the right person for the job, but if can find something you have in common with them it really helps you to stand out in their eyes.
I may have rambled a bit there, sorry.