5 Ways to Make Sure you Ruin Your Job Interview:

Chew gum – even if it is hidden in your cheek, it will be detected and not appreciated. Same goes for a sucking candy.
Put your feet up and relax – taking the liberty of making yourself at home, reclining and leaning in your seat, is a sure way to avoid being asked back for a follow-up.
Leave your resume at home – if the interviewer can’t locate your resume let him know to check his email – that will make an excellent impression.
Ask the interviewer if you could take a look at your own resume – if the interviewer asks about a specific experience listed on your resume answer with “Let me see that thing a second, just want to jog my memory.”
Talk endlessly – rambling, whatever the reason, is a sure fire interview spoiler!