My long time experiences as both interviewer and interviewee for IT jobs tells me that the most important things to do as interviewee is to be and feel comfortable, converse like a normal human being and be honest about your skills and abilities.

People in general tend to treat differently other people that can keep their cool in stressful situations. Interviewers subconsciously seek signals about candidates and being easygoing in conversation and radiating the aura of honesty and pleasantness may be as much or even more important than technical skills. You can train people to do their jobs but you can't change their attitudes toward other people. Single "difficult" personality may poison entire team and this is what good interviewers/managers try to avoid.