I've learnt to always have my nails shaped and polished, wear heels, love dry shampoo (makes my hair bounce), accessorise and never leave the house without make-up. Simple things which make even jeans and a vest top look good.
Also to avoid cheapy, cheapy clothes. A few well made pieces are worth a whole wardrobe full of tat.
Took me a while to get into the swing of it though.
DH who is French says the colours, the very unfortunate dresses in very in yer face colours (like bright yellow) with jeggings, wearing a cardigan... Apparently wearing large blocks of colour screams British. Flower patterns which aren't tropical flower prints so tiny flowers or rosebuds also scream Brit. Bright colours, pastel colours, colours in general. Definitely no blue, green, any cool colours. 'Proper' looking dresses (a line, block colour not black) and dresses which belong at garden parties.
He just went through my wardrobe to illustrate this point.