I think it has a lot to do with not wearing random items that don't seem to have any connection with each other. So say if I was wearing a black top and cream trousers, I'd try to wear something cream or black and cream near my face (necklace, scarf, earrings or hair accessory if your hair is long enough) so it looks as if you put them together intentionally, not just took the first things that came to hand.
Also, it's very rare for me not to wear either black, cream, taupe or denim blue on at least one half of my body. They are my favoured neutrals - yours might be navy or dark brown, but you'll know which colours are basics for you. If I'm wearing two different brighter colours, it becomes even more important to bring them all together with another item that involves both colours. I read an article once that stressed the importance of the "third thing". They meant that you never look really finished if you just wear a top and skirt or top and trousers, you need something, whether it's a jacket, cardigan, scarf, necklace or maybe even something as simple as a belt, to make it look like a "look" and not just "clothes". I'm not quite sure what you're supposed to do if you're wearing a dress, as I rarely wear them.
An idea I liked from a different article was "all neutrals and one statement item". This could be an item of jewellery, leopard print shoes or perhaps one bright colour in the form of a cardigan or a watch.
You don't necessarily need to wear lots of make-up, but do wear mascara unless your eyelashes are naturally dark brown or black. I know one person who always wears red lipstick (the rest of her look is quite simple) and it instantly says that she's made an effort. I've been trying to do the same (with a slightly more subtle shade that suits me better) but it hasn't become an automatic habit yet. If you can find a perfume that suits you and isn't overpowering, you could try putting that on when you're going to work too, to signal to yourself and others that you're on duty now.