Rum.
An amount of nervousnous is good, it shows you give a shit. I calm myself down so much in advance that i always come accross as not caring :/
The best thing you can do is be realistic about it, if you dont get the job its gonna be some good practice on the interview, if you do get it then woohoo. Make sure you actually believe you can do the job before you go and that youre good enough for it, so that you can answer honestly and openly without fear of problems. Ask yourself some practice questions, so when they ask you things youll have things to talk about.
Ultimately though like with any connection of 2 people, if you get someone who you remind of the person who shot his dog, youre fucked before you start, so dont take it too personally if it goes wrong or you couldnt make friends.