There are a number of things an interviewer notices in the first ten seconds of your meeting, and those ten seconds can have a critical impact on whether you get a job offer or not. Here's a quick guide to what they see.
Attire
Most interviewers expect a suit (very creative fields and high tech fields are often exceptions), and expect women to be wearing a suit or dress. If the candidate is applying for a job in marketing or public relations, a more fashion-forward image is expected. Things that make a bad impression include frayed cuffs, missing buttons, and scuffed shoes. And the interviewer will notice. Nearly all interviewers agree that it's more important for your clothes to be in good condition than it is for them to be fashionable. Many interviewers also notice whether female candidates are dressed in an extremely feminine or extremely masculine manner. (It may not be fair, but the tradeoff for added wardrobe flexibility is added scrutiny.)
Grooming
Make sure your hands and fingernails are clean, smooth, and well-groomed. Your mom was right—posture is important, so stand up straight and carry yourself with confidence. If you're female and wear nail polish, make sure it's not chipped or peeling. Messy hair is also a big negative, as is excessive or sloppy makeup. Remember, your attention to detail in grooming yourself reflects (in the employer's eyes, anyway) how you'd attend to detail on the job.
Bag or Briefcase
Many interviewers will notice what, if anything, you're carrying. As a candidate, you're going to both give and receive information in an interview. Don't fold your resume up in a pocket or purse—it looks sloppy and unprofessional.
Handshake
A handshake is a good indicator of how confident you are. It should be firm (but don't crush the person's hand—that's a huge mistake!) Give a full handshake, not just one where you grasp the other person's fingers (this is a mistake a surprising number of people make when shaking a woman's hand.)
