There are many factors that make a successful interview, but by far the most important is preparation. It seems simple, but it's not necessarily easy. Preparation is not only being able to answer the standard questions, but also being able to convey articulately how outstanding you would be in the position and the organization. Like most things, the better prepared you are, the more likely your success.
Knowing all the details of your cover letter and resume is crucial; make sure you can demonstrate or expand on the examples you have highlighted. Be familiar with the job description, the organization and the industry. There is no substitute for good research. Learning more about your prospective employer also gives you the opportunity to formulate questions about the company
I find it fascinating that, regardless of where people are in their careers, most still cringe at the thought of hearing the words, "tell me about yourself." This kind of question is actually a gift because it’s your opportunity to show off your very best self. To take full advantage of this opportunity, it is important that you don't just "wing it"; rather, come prepared to clearly articulate the claims you make in your resume and cover letter, and offer positive examples that match the employer's criteria. Given that you are the expert on you, this is the chance to show the employer how you are a great fit. I think of it as the 3-point plan: showcase your skills, demonstrate your experience and reveal your enthusiastic personality.
It is unlikely that you will be able to prepare for all the questions you will be asked, but you will often be asked standard questions like "Why should we hire you?" and "What is your greatest strength/weakness?" The Interview Question Database in the community is a great place to start researching general, skill-specific, and discipline specific questions you may encounter in an interview, as well as perspectives and tips on responding. When formulating your answers, ensure that they are clear, concise and your own. I would also recommend that you prepare some questions for the employer. A strategy I have shared with some people is to write out the questions you want to have answered on a post-it note or something similar that will easily fit into your pocket or purse. Include some general questions like "When will you make your decision?" and "What is the anticipated start date for this position?" It is likely that the employer will answer your questions during the interview, but when it is your turn to ask questions, check your list and ask an appropriate question.
If preparation is the key to successful interviews, then the key to successful preparation is practice. The more comfortable you are at answering the standard questions, the more confident you become in handling the more difficult and unusual ones. Like riding a bike, the first few times are awkward and uncomfortable, but the more practice you have, the more competent and proficient you become. The best way to practice is to find someone who will give you honest and constructive feedback, and to be open to that feedback. Then… practice, practice, practice!