Make a positive first impression. Smile be positive and warm. Don't be negative or critical. Have positive and interested body language.
Research the company their services, project portfolio etc. Get a brochure or visit their website and read it. At an appropriate time during the interview, make reference to something about the company that you have found out.
Listen to the question asked and answer that question and that question only. If you must digress, make sure you clearly link to the question asked.
Let them run the interview their way - even if you think they are doing it badly.
Remain positive and answer their questions, giving them the information they want.
Keep your eyes on their eyes. One of the most common breaks of eye contact occurs when people are thinking of an answer - that's no excuse. While you are thinking, remain attentive to the person you are speaking with by maintaining eye contact.
Travel light. If you have the day off and intend to go shopping, do it afterwards. Going to an interview with as many bags as a pack mule makes you look cumbersome.
Frame the answers to your questions in their language. Every industry has a favourite lingo. If you are crossing industries use their 'buzz words' instead of your old ones to show you have done your homework and can relate.
Towards the end express your interest in the position and give them genuine tangible reasons why you are interested. Be positive and confident as you ask them for some feedback on your suitability for the role. Finally reinforce your interest and ask if you can be taken to the next stage.
Relax and be positive. They are interviewing people because they need someone. Most interviewers are looking for reasons to employ people.