The key things to remember in an interview are-
* Keep calm
*Talk when asked something, dont let nerves take over and babble about meaningless subjects
*The person doing the interview is just a normal person, not a monster or a total asshole or bitch
*Involve yourself in the conversation in group interviews, but dont dominate it completely, let the interviewer do that
*In hypotheticals, just use common sense. When they ask "What would you do if a customer was complaining about the lack of customer service in the store?" dont answer with "I dont know" or "Palm customer off to boss" answer with "try to calm the customer, allow them to tell you what happened to give them this view, ask the customer if they want to write out a complaint form, ask if THEY want to speak to a manager, then, if they do, call a manager or supervisor."
*Be happy! A kid sitting in the interview room looking scared shitless and pissed off at having to be there is obviously in the job only for the money and wont be a good employee... and those sort shit the interviewer to tears! Smile, laugh at jokes, even make small ones yourself. Dont be daunted if the interviewer seems like a bitch, they have lots of bad days and are on a rushed timetable.