Everybody is going to have their own style that they prefer. So: these things work for me, but may not be your preference...
Really good quality accessories (jewellery, handbags, belts, etc). These can last for a veryy long time and it's worth spending money on them. They make all outfits look much better. I have very nice, fairly plain gold jewellery, which I can (and normally do) wear every day with everything. It was expensive, but the price per wear is miniscule now. Once you've got it, you've got it for life. Junk jewellery rarely looks good imo.
Clothes which have a deliberate shape and structure. Not things that just hang there (like most T shirts do, for example). A good, structured coat or jacket (in particular) will make everything else you're wearing look 100% better, and in the UK you can wear coats and jackets the majority of the year.
I tend to avoid black, especially for coats, handbags, belts and shoes. It can often look basic and cheap. It's not a hard and fast rule for me, but it's generally better to go for a different neutral colour. (I have an olive-green handbag with tan straps which is currently my favourite and goes with a surprising number of things).