At the very least, this means checking out the company online and reading any "About Us" or "History of" pages.   If you want to be even more educated, read up on their stock price or any appearance of the company name in the news lately.  Pay special attention to the company's mission statement and future goals, since you can focus on answering questions in a manner that will address these concerns. Knowing about the industry in general is also a good idea. The more informed you appear, the better the interviewer will think of you. If at all possible, ask who will be conducting your interview.  Knowing who to ask for, what his or her job title is, and how to address him or her will give the interviewer a more positive impression of you. Learn from other employees, if you can. Read any articles that you can find about the company and talk to current employees or others in the industry who know more about the company. You do not need to know it word-for-word, but it wouldn't hurt, either.   Knowing what a company wants will allow you to pitch yourself as the person who can meet those wants.  If there is a job description posted online, make sure you read each requirement and responsibility. Be ready to address each one and to talk about past experiences or current skills that address each point. An interviewer might throw out a question or two that you will have no way of anticipating, but in general, most questions asked at a job interview are pretty much the same. Write out at least five to ten potential questions you might be asked. Practice giving answers that address your career goals and qualifications.  "Tell me about yourself." The interviewer is essentially asking you to provide a professional overview of yourself. Briefly describe your past work experiences, how you got into the field, and any other information that describes your connection to the industry. If you mention anything personal, keep it brief and tailor it to demonstrate your overall character. "What is your biggest weakness?" Give a genuine interest, but phrase it in a way that dampens the effect. Also provide a quick explanation on how you have learned to deal with it. For instance, if you have crippling shyness, you can answer, "I tend to have trouble communicating with people at first, but as a result, I've learned how to observe people more thoroughly so that I know how to best interact with them after the first few encounters." "Where do you see yourself in five years?" This question can be difficult to answer since you need to seem both motivated and stable. If this is a company you plan to work for long-term, make sure that the interviewer knows this, but describe your desire to contribute and advance within the company. If the job is only temporary, say so, but make sure you describe what you aspire to reach after leaving the job and what you hope to have contributed after you go. "Why do you want this job?" This is your chance to show off your knowledge of the job description and the company. Show how the company's values align with your own. If it is a fast-paced company that hopes to advance quickly, express a desire to advance along with it. If it is a company dedicated to a cause, describe your passion for the same cause. At the end of the interview, the interviewer will probably ask you if you have any questions of you own. Never shrug this off, since it could be interpreted as a lack of interest on your part. Ask questions about office culture and questions that clarify what your job responsibilities would be. Also find out how soon you can expect to hear back from them. As a general rule, wear clothes that are one step more professional than the position demands.   Make sure that all your clothes are washed and ironed for the interview so that you look your best.  If the employees wear casual clothes, show up in business casual. If the employees wear business casual clothes, come to the interview in a full suit. If the employees already wear fully professional attire, though, then show up in the same.

Summary:
Research the employer. Know who's interviewing you. Memorize the job description. Practice answering some basic interview questions. Prepare your own questions. Prepare a professional outfit.