The key to appropriate interview attire is to keep things simple. Do not wear anything that may distract the interviewer. Your appearance should be professional and tasteful, yet completely unremarkable.
Make sure you're clean shaven.
Keep shoes well-polished and clean (don't wear military/uniform shoes) with dark socks.
Wear a belt that is in good shape and free of scratches.
Invest in two suits. Keep the style conservative: dark color (e.g., dark grey, blue, or black), solid or with subdued patterns. Single- or double-breasted suits with two or three buttons are acceptable. (Two-button jackets are the current style.) Wool is always a good choice, but any high-quality fabric will do.
Wear a high-quality white shirt with a straight collar.
A good-quality tie with a conservative pattern in blue or red is always a good choice. Tie it with a double knot (when tied, the point should come down to about the top of the belt buckle).
Ensure your dark socks will not allow any skin to be exposed when crossing your legs.
Wear modest jewelry (no more than one ring per hand), if any.
Wear a conservative wristwatch.
Wear contact lenses rather than glasses. (If you must wear glasses, wear a conservative style.)
Carry a handkerchief (not in your breast pocket).
Keep your nails neatly trimmed.
Be conservative with cologne (if you wear any at all).
Wear a lapel pin or miniature medals.
Wear tie clip or pin.
Wear a collar stay.
Wear bracers/suspenders.
Wear a vest (not even a matching vest that came with your suit).
Wear cufflinks/French cuffs
Wear military-issue shoes.
Wear military-issue glasses.