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...
Denim Jacket
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
i own way too many pairs of patterned tights, but they are a really good way of making a previously plain outfit more interesting with stripes, polka dots, floral designs and many more. i particularly like the henry holland suspender style that is popular at the moment.
Dr Martens
although slightly pricey, a pair of dr marten boots are worth the price as they are very versatile and wearable with most outfits for an alternative look, not to mention comfortable and a pair of these would last a lifetime. there are many styles and patterns available from floral to brightly coloured patent boots. stockists include sole trader, but if you want more choice i'd suggest camden market.
Denim Shorts
a must for every wardrobe, there are also many variations to suit most shapes and styles; high waisted, boyfriend, cut off, short shorts and many more. they can be worn with heels or dressed down with a band tshirt and converse, which ever suits best.
Ballet Pumps
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.
Statement Jewellery
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.
Red Lipstick
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 :)