It’s hard to imagine being over-prepared for an interview, but remember your experience and background constitute most of your preparation.  Of course you need to learn as much as you can about the prospective employer and the job.  To help here is a convenient checklist courtesy of the Contra Costa Water District.
They add another suggestion to the typical list: relax, and be yourself. Companies don’t waste time interviewing people they don’t think will do well in the job, so follow these steps, and remember someone thought enough of your qualifications to take the time to meet and learn more about you. If you’ve done your homework, there is no need to be nervous.

Before the interview:

Do plenty of advance research about the position and department for which you are interviewing.  This will allow you to discuss how your skills may specifically benefit the organization.

Anticipate questions you may be asked. Have a friend conduct a practice interview with you, so that you can answer these questions aloud.

Develop some questions that you can ask the interviewer. This shows that you have taken the initiative to learn about the organization, and that you have a genuine interest in how it works.

Dress appropriately.  Be well groomed.  Keep in mind that good posture and body language are important.

Be prompt on the day of the interview. Remember that you may need extra time to allow for traffic and to find the right building and office.  If you give yourself that time, you probably will be much calmer going into the interview.

During the interview:

Be ready to talk about your past experience, skills and abilities. Provide details, and discuss how these skills can enhance the job for which you are applying.  Don’t assume that an interviewer will automatically know that you are the best candidate.

Have confidence in yourself. If you’ve been chosen for an interview, it’s because you are qualified.

Don’t feel as though you have to immediately speak up when asked a question.

It’s all right, before answering, to take a moment to organize your thoughts.

If you do not understand a question, ask the interviewer to repeat it.

Finally, just relax. Remember, you’ve been called for the interview because you have specific skills to offer the organization.  This is your chance for you to talk about a subject with which you are very familiar — yourself!