Proportions are my first priority. I dress according to proportion (which is what I think gets all the compliments, aside from fit). It's a matter of trial and error but, like Diana, I just know. I always strive for an hourglass silhouette (or else a rectangular one), because they are the most proportional shapes, which is what *I* go for.
I also pick dresses over skirts and pants all the time because they make proportions easy for me. Otherwise I have to deal with color combos and waistlines and it's a pain.
Note that I am naturally an hourglass (short torso and long legs though, like Diana but with a bigger bust and a wider shoulder-line), I think, so any color blocking emphasizes one side of my body (and thus breaks proportions).
(I am the same way when taking photos, composing an image, even placing things on a shelf. It's one of my many quirks, this necessity to always even things out)
