Decent suit, watch, cuff links.
Firm hand shake
Thank them for their time.
Helps to show an interest/passion for what the company does, doing some research is important. Find out company history and where they may aim for the future.
Also research the people interviewing you on LinkedIn - i did that recently for my interview at McLaren, found out the guy has a similar career history to myself, similar interests etc, got on well and spent 2.5 hours there getting a one to one tour of everything.
Also know they need you just as much as you need them, give them a good interview back asking a load of questions.
I find it helps to take a folder with some paper in. Few company facts written down along with questions.
Just be yourself too - alot of employers like to see just genuine, hard working people who aren't social retards
Good luck!