In an interview? You first make a good appearance, speak clearly, use your best manners, and look into the eyes on the one interviewing you. If you have references, be ready to hand them to him.Be prepared. Talk slowly, be attentive to what is said and asked of you. Smile often and only ask questions that are pertinent to the job. You might do research on the job to know ahead of time of what will be talked about. You will not be forgotten for taking an interest ahead of time. Be willing to work extra hours and holidays and when done thank the person for his time.
I went into an office once and said I wanted to see the boss with the truth. The secretary gave me a hard time and then finally she called the boss. He came out and said what is this truth you are here about. I said "The truth is I need a job." and he hired me.
I suggest you get on the companies web sites and have knowledge of thier "culture" and operations beforehand. Then convince them you are a good fit. Smile, have good eye contact and yes, send them a thank you for the opportunity to meet them and for the interview. I also suggest you pay the place a visit beforehand if possible and observe how people dress. If they all wear professional attire (suits) - wear a suit etc. good luck!