first and foremost, turn off your cell phone (off as in off, not vibrate etc) before going into the interview
get rid of all your online junk, fb, twitter, youtube and all other things that show the true garbage you (if thats the case)
do not ask about 401k, vacations, sick days, or talk about bullshit on how every third weekend you help grandma change diapers. Regardless how emotional the bullshit you do is to you, no one else gives a fuck, they are a business not a homeless shelter.
SHUT UP and listen, dont interrupt, ask what is answered, DO NOT make jokes, the fact that a few facebook crackheads told you you have a good sense of humor means shit.
Last but not least show them that you understand what you are being hired for (that is if you understand) be specific as to what you can do, how excited you are about doing it and go further by saying i'd take this job even if the pay is less.
Only pick jobs hat you are passionate about, if you pick em only because of pay, within a few years your resume will be a few pages long.