You should always have the wardrobe basics that allow you to go anywhere.

For guys, this is easy.  If you have decent shoes, khakis, jeans, a white long sleeve dress shirt, a solid color tee (preferably white, black, or silvery gray), a muted tie, and a jacket, you can go absolutely anywhere.

Women need good pumps and some nice flats.  For informal things, well-fitting jeans, a solid button down shirt, a cardigan.  For more formal things, I think a sleeveless black sheath is essential.  A jacket completes the outfit for business.  For an evening out, a long sparkly scarf or a patterned shawl can make the outfit.  A simple necklace and stud earrings and you are done.

Of course, these basics should always be clean, pressed, and in good repair.  You never know when a special invitation may appear!