Attire: For men, a well pressed shirt and tie under a suit should suffice. Women however, have a choice between skirts or pants when it comes to suits. Although it is often assumed that skirts work better, I scored my first interview wearing pin-striped pants with a matching suit jacket. This shows that ultimately it is what you feel most confident in that truly works. A dress with a matching suit jacket is another option for women. If the field that you are interviewing for is not too formal, a cardigan over a one-color sheath dress 2" or more below the knees would be appropriate. Note that for some professions, dark colors are a must (e.g. Finance). If you prefer colors other than the classic black, choose grey, navy or the pin-striped variety. Attempt to dress as comfortably as you possibly can because this will make you feel more relaxed during the interview.
Shoes: Despite being less noticeable than your attire itself, shoes do matter to interviewers. Men should wear polished matte shoes, and men's socks should be understated and dark colored; this is important for when you cross your legs! Women should pick closed-toe shoes over flimsy sandals. Heels should be an appropriate height - 2 ½" tall or less. Pumps work just fine in any color whereas 4" stilettos are a no-no for all professional lines. If stockings are part of your attire, make sure to check for runs.
Grooming:"Neat and tidy" is the key. Gel is appropriate when used in controlled amounts. If you have long hair, it is recommended to tie it up. Keep hair away from your face, and tucked behind your ears. This will prevent you from constantly moving it away from your face while you talk.
Location/Occasion: Depending on the location of your interview, your attire will vary. If you have a lunch interview, choose clothing that allows you to maneuver your arms around easily. Stiff clothing may restrict movement and as a result create clumsiness. If you are interviewing for an internship position, formal dressing without a suit should meet the requirements. Match a smart shirt and tie with well pressed pants. Keep the shirt tucked in at all times, and you're all set.
Season: During the winter season, wool blend suits will keep you warm under a winter coat. During summer, lighter materials will prevent you from sweating (a sign of nervousness).
Fragrance: Cologne and perfume should be used in minimal amounts. Try to avoid smoking before the interview, as some interviewers may not tolerate the smell. If you do, have a mint before you step in.
Carry-ons: Briefcases are appropriate for both men and women. Studies have shown that purses make women look less professional, so if you can avoid carrying a purse, do so. Instead use a leather binder to keep your pens and important documents. Get one with a pocket so you can slip your credit card and phone in it as well.
Confidence: Lastly you should wear a confident smile. Confidence will get you a long way in an interview. Without it, tips 1-7 won't even matter.