You only get one chance to make a first impression!  It is estimated that nine out of ten employers will reject an unsuitably dressed applicant without a second thought.  Like it or not, your outward image, attitude, confidence level and overall delivery are directly affected by the clothes you wear.  The respect you receive at an interview is proportionate to the respect your visual image earns for you before you have a chance to say a word.  With this is mind, here are some tips for dressing for success on an interview:

Men
Single-breasted dark suit (preferrably black or blue)
A starched, white, long-sleeved, 100% cotton broadcloth shirt.
Highly-polished, good quality, leather shoes (black preferrably)
A semi-conservative silk tie
Socks that compliment the trouser color by extending into the shoe

Women
A black, gray, or navy suit (A skirt suit is the universally accepted look)
Silk blouse in white or ivory speaks for authority
Shoes of good quality leather (professionally polished with attention to the heel)
A business watch with a leather or metal band
Jewelry should be of good quality and limited to a few pieces