When H1 and I became well-off, we were painfully fashionable. You know, we owned designer stuffs and were friends with the designers ... and went around in baggy black tops, ripped jeans and DMs. If you knew your fashion you'd be able to tell the jeans had been customised by Paul Smith and the DMs were limited edition, but that's all part of it, innit 
Anyway. So we bought a house, designerfied it very fashionably, and sallied forth to buy a designer sofa at a stratospheric price. Sofas not being our usual remit, we went to Posh Shops (rather than galleries and ateliers, darling). It was hilarious. The supercilious, pretentiously expert staff clearly weren't pretentious enough to discern our exquisite tastes. They saw a young couple in ripped jeans, sneered a bit and ignored us. We had to practically assault them to get service. When we chose a sofa, the jerk actually laughed when we said we'd take it. Changed his tune pretty fast.
We did go back to the same shops later, buffed, polished and wearing the showcase designer stuffs. This time the staff saw us our clothes coming and welcomed us at the door, all Sir and Ma'am. We wasted their time for a bit, then told them we weren't buying because they'd been so rude to us a few weeks ago.
There's no moral to this story except that it's a mistake to judge on outward appearances, especially if you're doing so pretentiously. As others have posted, the really really posh don't dress up; they've usually got fields to check and dogs all over the place, so wear old clothes that can stand the battering and keep out the cold.