• Being unprepared for the interview - you should always prepare thoroughly before any interview (this will also make you feel more confident at the interview). Reach early, imagine that you have already been selected . 
• Poor/limp handshake - always a bit of a no-no. Offer a firm hand but do not press his too hard.
• Saying unfavourable things about previous employers - the employer will be wondering what you will say about them when you leave their employment.
• Not being able to communicate clearly and effectively. If you stammer then talk slowly.
• Being aggressive or acting in a superior way - nobody like this so please don't do this.
• Making excuses for failings. Say you made a mistake and want to correct it as early as possible.
• If you feel you have not done well apologise and request for a feedback.