The biggest mistake in interviewing is not being fully prepared. Understand that interviewing is a skill; as with all skills, preparation and practice enhance the quality of that skill. Preparation can make the difference between being successful at interview or not. Every interview is a learning experience, so learning that takes place during the preparation and actual interview process is useful for future interviews.
Initial preparation requires recent assessment of skills, interests, values and accomplishments; a re-assessment and updating of one's CV; and research on the targeted company/organisation and position.
Preparation Before Interview Process
Self-assessment: Reassess current skills, talents, abilities, strengths, weaknesses and interests. In addition, re-examine accomplishments and achievements, particularly those that may be relevant to a prospective employer.
Update your CV: Remove all irrelevant information. Use functional headings to help focus the reader on what you have done and what you can do for the prospective employer. Be absolutely certain it is error-free.
Prepare possible answers to questions that may be asked: Demonstrate your competence, experience, knowledge and abilities. Make a list of all your past significant achievements - expand on the ones you have highlighted in your CV.