Research your prospective employer (the company). Research the duties of the position you'll be interviewing for.
Confirm that you know the precise location and the route to get there.
Wear proper attire. Have an idea about the company's policy on facial hair and keep all hair neat and trimmed.
Rehearse the answers you'll give to common interview questions; e.g. Where do you see yourself in 5 years? Tell us about a problem you had with a manager at a previous job and how did you resolve it? Why are you leaving the company you're with? etc.
Know your availability and when you will be able to start, if necessary. Depending on the job type, you may want to negotiate salary.
Bring a copy of your resume on resume paper, and also bring any supporting documents such as the school transcript(s) if you're a student, for example.
Adopt a sober mentality before you take the interview and do not add unnecessary information during the interview.
Do your best and communicate your abilities. Then let them decide if they want this person or not.