I know someone who always looks put together and classy, and I've realised it's because she:
Has shiny hair (not blonde btw, no greys or roots showing) also not always straight. Very subtle lowlights that give it dimension without really being noticeable unless it's in direct sun.
Wears combinations of black and taupe/beige. So all her clothes go together and look like an 'outfit'. I've never seen her in blue jeans, for instance.
On the rare occasion she does wear something that is not black and/or taupe, it is a single item and colour, like an unpatterned dress.
She's slim and toned. Her clothes fit. Nothing digs in or stretches or looks baggy.
She always has make up, but it's entirely neutral - foundation (excellent shade, matches perfectly), blush, possibly highlight and contour - but I can't tell! - a neutral eyeshadow that's a shade or 2 darker than foundation, mascara, tightlined eyeliner.
She always has a slight tan, so the black clothes don't make her look washed out.
Accessories are plain and gold, simple huggie earrings, fine gold chain, even hair bands and clips are either black or gold.