I think quality of fabrics, colour, etc matter less than the silhouette. The French are obsessed with this, and with good reason.
Think about the shape you want to present to the world and arrange your wardrobe accordingly.
Avoid:
Long top layered over long skirt
Baggy top over baggy trousers
Short top and short skirt
etc
If the top half is fitted, go looser on the bottom half, and vice versa. I really think this is where a lot of people go wrong.
Also, sports wear should only ever be worn when actively playing sport, or on the way to and from the gym. Fleeces, tracksuit bottoms and trainers have no place in your wardrobe if you want to look pulled together.
Finally, read Vogue religiously (but shop on the high street)
Disclaimer: I often look like total crap, so I'm not preaching - I aim for this but do fail.