Oooh, great thread!
I completely concur with the others that proportions can make or break an outfit. I am very mindful of whether to tuck or untuck my shirt, and it often depends on what I'm wearing on the bottom and what shoes I have on. If I will be wearing heels, I often untuck, because the heels make my lower half longer, and the top half needs to be extended somewhat to balance it out. If I'm wearing a maxi skirt, I often untuck, because the maxi extends all the way to the floor and is usually somewhat too high-waisted for tucking. I have found that I like to wear flats with my maxi skirts to avoid an awkward extension of my lower half (I'm already long legged, and maxis just emphasize it).
I also have to say that proper fit of the items is crucial. Fluid fits have to be just so, so that they don't look frumpy or ill fitting, and fitted items have to be nicely fitted but not look like you bought a size too small. I continue to be amazed at how good I look when an item fits me perfectly. (It's not you, it's the clothes, as Angie says.)
I also think that balance in the silhouette is crucial. If you are wearing volume on top, oftentimes a more fitted bottom works, and vice versa. I do know that sometimes people wear volume on volume, but I assume there is some trick to this that they need to share (I can't do it).
The last item I would suggest doesn't really have to do with clothes but has to do with personal grooming. Sometimes some extra attention to hair or nails or makeup can really take a regular outfit to stunning. Or if the outfit seems to lack something, a piece of statement jewelry can fill it out.
