greet them with a handshake and a smile. first impressions are VERY important.
do NOT be late. if you are, phone ahead and let them know.
be friendly and humble, dont try and act cocky like you know everything even if you do, but demonstrate your knowledge. your manners and the way you come across is very important.
know your basic stuff about the company, dont go in without doing any homework about the place or else u will look silly.
BE YOURSELF. try not to get nervous, they are not above you they are human too and were once in your same position sitting in the interview chair.
talk slowly and dont jump from point to point mumbling your words like some people do, communication is very important. take your time theres no rush.
dont kiss ass. they can tell when you are doing it and they hate it. try to be genuinely friendly, but not over friendly because its fake. if you can make them laugh its good but again dont over do it, just try not to be too serious and come across as a boring old fart, your personality and vibe you give out also sells you. just be friendly an smile but dont over do it an try and come across as genuine (if you arent lol).
go into the interview believeing that it wll be THEIR loss if they dont hire you, and project that vibe onto them into the interview..this doesnt mean be cocky and a smartass, just believe in yourself.
try to steer the conversation onto experiences youv had, eg building a project, something technical you can talk about, this way, hopefully you KNOW what you are talking about, and you can use up the time on this, if you are quiet, they are more likely to ask you more technical questions increasing the chance of you getting one wrong. if they do ask you something you dont know, be honest about it if you dont know, dont try and make something up because it will make you look like afool. not everyone knows everything.
thank them for the interview with a smile and leave
oh, and think of some typical questions they might ask you and practice answering them, but dont try and memorise exact replies because it will come across as fake an scripted, just have bullet points of things you want to say, and just talk about it naturally from the top of your head..as if just talking to one of your friends...this is much better in my experience.
hope this helps