Just like a good pair of jeans, a denim jacket that belongs over everything, a LBD, a LWD and a leather jacket, the denim shirt is a wardrobe necessity. It has the power to tone down an outfit that could be overkill (hi Anja Rubik), it adds a little structure whilst not parading as an uncomfortable starchy blouse and works perfectly on those days where you feel a little over indulged and uninspired.
These are a couple of my favourite:
Rag & Bone/Jean Trail Shirt
Express Dark Wash Denim Shirt
Gap 1969 Western Shirt
Alternatively you can go vintage to find that perfectly worn-in shirt if the thought of shopping at Express is too terrifying to fathom.