I'm coming to the conclusion that there's a couple of rules to be observed, above and beyond the personal grooming ones.
Rule 1, I think, is to work out the correct proportions of accessories, lengths, widths etc for your body size and shape. An example, would be if you've got short legs and a long body, and you wear jeans that aren't the right length - so you look as though you've got even shorter legs.
Rule 2, I think, is working out the colours that suit you - above and beyond thinking "oh blue always suits me'. It's getting that right shade of blue, and working out that in the winter when you might be paler, or when you're older, and your skin tone/hair colour has changed, that that blue needs to be different too.
And rule 3, has to be having clothes that fit. I used to work in the Far East, and a colleague once said that she thought I was really well dressed. I was a post-baby size 14 and was having to have a lot of tops etc made for me, as I was way too hefty for local clothes. But they all fitted me, and that's what my colleague noticed and why she thought I was well dressed.
Just a thought.