Nothing wrong with supermarket clothes but I know what you mean. I find that I often buy something I didn't particularly need or want, just because I noticed it during the weekly shop. If I'd been specifically looking for something on the high street I might have been more discerning.
I am trying not to buy lots and lots of cheap things and instead investing in fewer, more expensive items. Not ridiculously expensive (almost always in the sales), but good quality.
So I would say...
A good coat, really good quality (coats take a hammering so quality is important). I bought a Jaeger quilted one in the January sales 4 years ago and it still looks immaculate. It was £120 (normally £250) which is very expensive for me.
Good boots (nowt wrong with Tu and Clark's!)
Good bag that you really like and is the style you're aiming for.
Some statement jewellery to set off your style.
My style is fairly classic - smart top or blouse and black skinny trousers or pencil skirt for work, skinny jeans and plainish jumper for home. I always wear a massive distinctive necklace or earrings, bangles, chunky metal watch etc. I like it.