Good underwear.
You can wear incredibly expensive clothes but the wrong bra - even though unseen - will give you a terrible shape, bumps in the wrong places and awful posture. Find a decent bra, buy it in three colours, and you can get away with cheaper clothes. Likewise knickers - ones that give you a vpl or that are forever disappearing up your backside will do nothing for your style and comfort levels. You need the right underwear for the right clothes. Knowing when a wonderbra is appropriate is crucial too. Cleavage that looks like it'll take a small child's eye out is definitely not an expensive look on the school run, but might work in the evening (I'm doubtful it will ever work but I might just be jealous!).
Good basic pieces is the next step.
Never buying anything in a sale just because it's excellent quality and vvvvv cheap
. There's a reason that cashmere sweater is reduced to £15, and it's usually because the colour doesn't quite work for anyone, or the length is just slightly too short. I have just culled my wardrobe of some lovely clothes bought in sales over the years - they'd make someone else look stylish/expensive but the colours are just too dark/strong/yellowish for me and will never make me look my best. I look great in turquoise and navy but need to be much more careful with teal.
I feel a bit of a hypocrite though as my wardrobe is definitely tatty atm and I'm overhauling it as my NY project. I am guilty of too many GAP t-shirts and sweaters since having DC.
As PP have said, grooming is also important. Brows make a huge difference, I feel desperately unkempt when my brows need threading or nails need doing.
It is easier to wear a wider range of clothes if you're tall, but I don't think tall necessarily equals stylish/expensive. I know a couple of tall women who tend to stoop, so things hang badly and they look less confident. I know one of them has real problems finding well-fitting clothes too. It's a case of knowing what suits you. I have several friends around 5'4" to 5'6" who always look 'expensive'; height isn't crucial. Yes, they are all slim.
Personally I think that wearing head to toe from one expensive shop/designer looks cheap. I don't really know why though. Chucking a e.g. Zara piece into the designer mix seems to liven it all up a bit.
Will watch this thread as I continue to chuck stuff into the charity pile!