There are many styling rules: the third-piece rule, the focal point, the wrong shoe theory, and so on. Some are pretty easy to implement, like the third piece, which I lean on frequently, and some feel too daunting when you're trying to get out the door, like the texture rule or the balance rule (balancing volume and fit).
Curious what works for you on the go! For me, it's tying something around the waist or shoulders, like a button-down or light sweater, and a pair of red flats for the "pop of color" trick.