here are a few staple items that pretty much make my wardrobe. they can be worn with most things or interchanged for different looks, but overall i think these items are worth investing in...
either worn casually with a t-shirt and shorts or slightly smarter with a dress, a denim jacket will suit most occasions. you could also wear it over a hoodie in winter months.
Black Patterned Tights
these are so comfortable to wear and you can get tonnes of different patterns and styles to wear with many different outfits. i have about 10 pairs myself, but i think everyone should own at least one pair of these, definately. they are relatively in-expensive and available from most good retailers.
a few contrasting or large statement pieces will instantly liven up a plain outfit, whether it be chunky bangles, an over-size necklace or head piece.
The Little Black Dress
a black dress can be worn over and over again to different occasions with different accessories for unique looks each time. there are many variations to suit all types of figure.
okay, not an item of clothing; but red lipstick is a total must. it can instantly add glamour and colour to a plain face, dressing up the most casual of outfits.
that's all i can think of for now. a few other of my own wardrobe staples include band t-shirts (i have tonnes of them), pretty floral dresses, a few pairs of heels, leopard print items, black & gold, tube skirts, a faux leather jacket & that's about it. if you have any other suggestions, i'd love to hear them! just post them as a comment :)