Suit: A two piece matched suit is always the best and safest choice.
But what if the job is in a non-suit-wearing work environment: Even if you would or could wear jeans on the job, or the work environment is outdoors and very non-suit, wearing a suit to the interview shows you take the interview seriously as a professional meeting. Dressing well is a compliment to the person(s) with whom you meet. If you think the industry in which you're interviewing would frown on a suit, or the interview will involve going to a work site where a suit would be inappropriate, look for advice through professional organizations, your professors who have been employed in that industry, and/or by asking the employer directly and politely.
Conservative colors / fabric: Navy, dark gray (and black for women) - are safe. Other color trends may come and go; avoid the extremes. Solids or very subtle weave patterns or plaids (the kind that look solid across a room) are safest. Wool, wool blends, or good quality micro fiber for women only, are generally the best fabrics in all seasons. Avoid acetate / rayon blends.
Dark socks, professional shoes
Cost / quality: You are not expected to be able to afford the same clothing as a corporate CEO. Do invest in quality that will look appropriate during your first two or three years on the job. One good quality suit is sufficient for a job search if that is all your budget allows. You can vary your shirt/blouse tie/accessories.
Details: Everything should be clean and well pressed. Carefully inspect clothes for tags, dangling threads, etc.
Additional interview attire specifics for men
Suit: A two-piece matched suit is always the best and safest choice. Don't combine a suit jacket with pants that don't match.
Conservative colors / fabric: Navy and dark gray are safe and are the most conservative for men. Black for men was once considered severe or overly formal, and may still be considered so in very conservative industries, although it is commonly worn by many. Other color trends may come and go; avoid the extremes. Choose a solid or very subtle weave pattern or plaid (the kind that look solid across a room). Wool, wool blends, or very high quality blends with natural fiber, are the only acceptable fabrics for a conservative men's suit.
Cost / quality: You are not expected to be able to afford the same clothing as a corporate CEO. Do invest in quality that will look appropriate during your first two or three years on the job. One good quality suit is sufficient for a job search if that is all your budget allows. You can vary your shirt and tie.
Ties: Tie styles come and go. Select good quality silk ties. Avoid fashion extremes, like character ties, in interviews. Notice what men in your industry wear on the job, at career fairs, at information sessions, when they meet with clients.
Shirts: Long-sleeved shirts, even in summer. Choose white or light blue solid, or conservative stripes.
Socks: Dark socks, mid-calf length so no skin is visible when you sit down.
Shoes: Leather, lace-up or slip-on business shoes, preferably black or cordovan. Invest in a good pair; even if you don't wear them daily on the job, you'll need them for other occasions and you should expect to get lots of years out of good shoes.
Belt: Black or cordovan leather, to match your shoes.
Facial hair: If worn, should be well-groomed. Observe men in your industry if you are unsure what's appropriate or are considering changing your look.
Jewelry: Wear a conservative watch. If you choose to wear other jewelry, be conservative. Removing earrings is safest. For conservative industries, don't wear earrings. Observe other men in your industry to see what is acceptable.
Details: Everything should be clean and well pressed. Suits typically have tacking stitches to hold vents - on the jacket back and on sleeves - in place before the garment is purchased. Cut them off if your retailer / tailor doesn't. And that tag stitched on the outside of your sleeve is not meant to stay there like a Tommy Hilfiger label - cut it off! Carefully inspect clothes dangling threads, etc.
Additional interview attire specifics for women
Don't confuse club attire with business attire. If you would wear it to a club, you probably shouldn't wear it in a business environment.
Suit: Wear a two-piece matched suit.
Suit - pants / skirts: Tailored pants suits are appropriate for women. Pants suits can be an excellent choice for site visits, particularly if the visit involves getting in and out of vehicles and/or the site is (or includes) a manufacturing plant or industrial facility. If you wear pants, they should be creased and tailored, not tight or flowing. If you are pursuing a conservative industry and are in doubt, observe well dressed women in your industry on the job, at career fairs, at information sessions, etc.
Skirt lengths: Much of what you see on television shows that masquerades for professional attire is actually inappropriate for a work environment. Your skirt should cover your thighs when you are seated. Showing a lot of thigh makes you look naive at best, foolish at worst. A skirt that ends at the knee when you're standing looks chic and professional. Longer skirts are professional too; just make sure they are narrow enough not to be billowing, but not so narrow that you can't climb stairs comfortably. Don't purchase a skirt or decide on a hem length until you sit in the skirt facing a mirror. That's what your interviewer will see. Ask yourself whether it will be distracting or reinforce your image as a person who looks appropriate for a business environment or gathering. High slits in skirts are not appropriate. A small back, center slit in a knee-length skirt is appropriate. On a calf length skirt, a slit to the knee to facilitate walking and stair climbing is appropriate.
Color / fabric: Navy, dark gray, brown and black are safe. Other color trends may come and go; avoid the extremes. Women generally have more options with suit color than men. For example, while a women could look conservative in a slate blue or light gray suit, these colors would be inappropriate for men. Choose a solid or very subtle weave pattern or plaid (the kind that look solid across a room). Wool, wool blends, and high quality blends and synthetics are apprpriate for women's suiting.
Shirt / sweaters: Underneath the suit jacket, wear a tailored blouse in a color or small print that coordinates nicely with your suit. A fine gauge, good quality knit shell is also appropriate underneath your suit jacket. Don't show cleavage.
Jewelry / accessories: Wear a conservative watch. Jewelry and scarf styles come and go. Keep your choices simple and leaning toward conservative. Avoid extremes of style and color. If your industry is creative, you may have more flexibility than someone pursuing a conservative industry.
Cosmetics: Keep makeup conservative. A little is usually better than none for a polished look. Nails should be clean and well groomed. Avoid extremes of nail length and polish color, especially in conservative industries.
Shoes: Should be leather or fabric / micro fiber. Shoe styles and heel heights come and go. Choose closed-toe pumps. Regardless of what is in style, avoid extremes; no stilettos or chunky platforms. Make certain you can walk comfortably in your shoes; hobbling in uncomfortable shoes does not convey a professional appearance.
Hosiery: Should be plainly styled (no patterns), sheer (not opaque), and in neutral colors complementing your suit. Avoid high contrast between your suit and hosiery color.
Purse / bag: If you carry a purse, keep it small and simple, especially if you also carry a briefcase. Purse color should coordinate with your shoes. You may choose to carry a small briefcase or business-like tote bag in place of a purse. Leather is the best choice for briefcases; micro fiber or fine wovens are also acceptable.