Watch your proportions. With the utilitarian chic of a denim shirt, you need to make sure your entire outfit has the right proportions to avoid looking oddly androgynous. If your shirt is fitted through the body, try a straight skirt in any length from mini to maxi, or a trouser-cut pant in a a touchable fabric, such as silk or suede. Pair skirts with a heeled boot and trousers with a platform pump. If your shirt is over-sized or tunic length, keep your bottoms slim with a fitted cropped pant, leggings or a sweet mini. If you wear a slim bottom, heels will take your outfit to the next level of fabulous.
Mix your fabrics. Though you can pair denim with denim, it's so unimaginative. However, a denim button-down paired with a suede pencil skirt and boots leaves no question about your feminine fashion skills. Even better, a fitted denim shirt over a sequin mini with metallic ballet flats or strappy sandals may bring you major mojo.
Follow the curves. If your button-down is oversized, a great way to make it feminine and sexy is with a hip-slung belt in soft leather or metal chain-link. Roll up the sleeves a turn or two and unbutton the neck as low as you dare or add a silk or lace cami underneath. No one will confuse you with the guy next door.
Accessorize wisely. The appeal of a denim shirt lies in its inherent masculinity, so when you style it, you'll want your accessories to have a bit of the femme fatale in them. Ropes of pearls, anything that sparkles or divine vintage touches, such as a velvet flower pin or flirty hat, takes your denim from basic to bombshell in seconds. Skip the clunky shoes and boots and make sure that your footwear has a bit of a heel, a bow or glam details, like sparkling beads or delicate crystals.
If you're lucky enough to find that perfect denim shirt that you want to keep forever, give it an instant upgrade by switching out the buttons. Comb through vintage shops or craft stores to find ones that you love and ask your tailor or dry cleaner to replace the old with the new. If you're handy with a needle and thread, do it yourself and save a few bucks.