In addition to an unprofessional appearance, here are six factors that could ruin your chances of a receiving a job offer:
(1) Being unprepared for the interview. Prepare, plan, and practice! In today’s tough job market, you MUST do everything you can to give yourself an edge… preparation is the key.
(2) Not being able to communicate clearly and effectively. This is important during the interview and on the job. Being nervous can really mess up your communication skills, so being well prepared and practicing what you’re going to say are always your best bet.
(3) Being aggressive, arrogant, or acting in a superior way. No one wants to hire or work with people who think they’re better than everyone else. Be careful with your attitude, even if you think you’re surrounded by incompetent fools. Being confident is good. Being an arrogant jerk is bad.
(4) Making excuses for failings. Your teacher never bought “The dog ate my homework!” and your boss isn’t going to buy “The finance department gave me the wrong figures!” In the grown-up world, you have to take responsibility for what you are responsible for! You’ll never earn respect by blaming others when things go wrong.
(5) Saying unfavorable things about previous employers. Even if you left a job because the boss was an egomaniac who took credit for all of your hard work, verbally abused you in front of others, and poisoned the plant on your desk, don’t say anything bad about him/her during an interview. When asked “Why did you leave your last job?” say 
something like “My manager and I both agreed that my advancement opportunities were limited there and obtaining another position was the best option for me and my career goals.”
(6) Having a poor/limp handshake. Why do people think you’ll be a lousy employee if you have a lousy handshake? That’s not really logical, is it? Doesn’t matter. It just turns people off and gives them a bad impression of you. So make your handshake firm and confident but not bone-crushing. (It’s not a competition to see who winces first!)