Dress professionally but don't go over the top either.
Have a few achievements and goals in your mind.
Plan for the simple questions like "How would you describe yourself?" "What makes you the right person?" "How would others describe you?" "What would your former managers and coworkers say it was like to work with you?" "Want to watch me take a shower?" "What's the thing that you liked the most about the last place you worked and the thing you liked the least?"
And then just be able to describe, politely, why they would be an idiot to offer the job to anyone else. Attention to detail, passionate about getting the job done right, willing to go the extra mile, embrace a challenge, etc etc etc.
But don't bullshit them. Whatever you say you really need to believe. If you're genuine when you say that strive to be in situations where you're likely to fail because you always come away learning something about the job and about yourself then that carries over.
And always have some questions in mind about the position, what their expectations are, chances for advancement, benefits, etc. My favorite is "Why are you hiring for this position?" Was someone fired, promoted, retired, or expanding?