There is no automatic win for interviews, but there are some things to do that bolster your chances. A few of these are obvious, some of these people don't really think about I've come to learn.
- Are you clean shaven? Is it relevant to the job you're applying for (e.g office job - clean shaven, construction work - acceptable but kempt)?
- Have you showered? Checked your nails for dirt? Brushed your teeth? Used deodorant? Appearance is extremely important. Groom yourself.
- As a follow on for the first point, does the clothing you're going to wear to the interview match the job? Generally any retail job involves smart-casual, office job smart, bartending casual and so forth. (If you're hoping to walk in and get a trial shift then and there, wear very neutral coloured clothes and closed shoes.)
- Do you have your resume, CV (cover letter) and any other required documents with you? Put them in a clean physical folder or portfolio and bring them.
- Do you know anything about the company you're working for? Though you don't need to bring it up personally, there might be questions regarding their company and how well acquainted you are from their end. Pretend to be interested.
- Have you worked out transport? From there you can make sure to be at least ten minutes early, preferably not longer than fifteen.
- Do you smoke? Don't do that an hour before the interview, at all if possible.
Make eye contact. Speak loudly enough that they'll never ask you to repeat yourself. Do not fidget. Keep your body language as open as possible without looking sloppy (Don't cross your arms if/when you sit down, keep your back and neck straight).
I think that about covers interview preparedness.