In big organizations interviewers are mainly personnel professionals who are expert in the art of interviewing, so expect the interview to be very structured to assess you thoroughly. In small organizations you are more likely to be interviewed by the owner who may not be a trained interviewer. Whatever be the case you need to be prepared well for the interview.
Before appearing for the interview do some basic research about the organization. Go through the organisation's web site and get some basic information regarding the organization, such as its turnover, key products / services, key competitors, new product launches, current leadership team et al. etc. You can score extra points by mentioning these during the interview.
You should know about the job you are interviewing for: job profile, what the job entails and why you would be good at it. You need to bring up why you would be a good fit for the position during the interview.
Be clear about what you want. This may sound elementary but most people are not very clear about their expectations. It is important to be clear about what you don't want as well. This will help you in the final negotiations. An applicant who is willing to take anything will usually get nothing.
Don't treat an interview like an interrogation. An interview is a mutual discussion. You also need to know about the job and the company. Asking meaningful questions about job profile, career paths, etc, projects you as a serious candidate. The employer also starts to view you as a confident and a focused professional.
Go with a positive attitude for the interview. Look at each interview as a learning experience. Start realizing that not getting a particular job is not the end of the world. Try to learn what you did wrong after each failed interview. Interviews are great opportunities for you to gain knowledge on how professional environments operate. Try to enjoy your interview and view the interviewer as just another person.