Learn about the company and know what they do. The most important thing you can do to prepare for an interview is to learn as much as you can bout the company, the person you are interviewing with, and the key staff members of the company. Read the company mission, vision, and values if available online. Research their competitors and be able to tell the employer why you want to work for them.
You should dress one step above what everyone else at the employer is wearing. For example, if you go to an office and most of the gentlemen are wearing dress shirts and slacks, then you should wear a dress shirt, tie, and slacks. Be sure to pick out an outfit the day ahead of the interview to ensure it is clean and unwrinkled. If you have tattoos, it is recommended to cover them up with your clothing if possible. Jewelry should be kept to a minimum. If you have multiple piercings, limit earrings to one per ear and keep them modest in size. Keep perfume/cologne to a minimum as some people may be allergic. Shoes should be clean. Men should have a trimmed or clean shaven face.
Stay positive and greet each person in a positive manner. The person you speak with at the front desk may have some weight in the hiring decision. Many companies ask the receptionist and others who come in contact with a prospective applicant for their opinion of the applicant. Each time you are introduced to someone be polite and shake their hand. Use your manners. Manners and simple politeness can sometimes win an employer over. Ensure each question answered is done so in a positive manner, do not state anything negative about any previous employer.
Search online for common interview questions and practice your answers.
Many employers will provide you with an opportunity to ask questions. Be prepared to ask a few questions related to the job and/or company.