Work clothes should not be too ill-fitted, in all senses of the word. This applies to your casual dress as well: jeans should be the correct length and shouldn’t be ragged at the bottoms; shirts should be covering all the necessary areas; and, gigantic sweaters that look more akin to ski weekend cover ups need not join you at work. Shorts? Let’s not even go there–unless it’s 90 degrees and the A/C is kaput, in which case, reassess your company atmosphere.
Wear items similar to what you would normally wear with the silhouette maintaining its professional look, and the other parts of the clothing speaking for its casual nature. Button downs in chambray or cotton are a nice change from starched dress shirts. If wearing jeans instead of pants, choose those that hit at your correct waist, like your usual dress pants would, and make sure your shoes fall somewhere between slip-on sandals and Vegas heels. Dresses shouldn’t be too festival-like, but also shouldn’t be too inappropriately fitted. If you wouldn’t wear it to meet your significant other’s parents at a casual brunch, don’t wear it to the office.