A defining characteristic of the well-bred haut monde is their superior intellect, cultivated by attending public English boarding schools and marked most obviously by their refined, expansive vocabulary. A gentleman speaking confidently with eloquence will be instantly regarded higher than one who blunders about speaking plainly like a savage plebeian. One of the greatest things you can do to improve your standing is to brush up on less-common words.
Words don't necessarily have to be ridiculously large to be a superior alternative to ordinary words. Find words that are more obscure, especially ones deemed 'archaic' or those with obvious Latin roots. Flamboyance in speech breaks the monotony and people will welcome you as a breath of fresh air in comparison to the proletarians with whom they are used to speaking.
I recommend a series of books by Peter Bowler for expanding one's vocabulary to include more eccentric verbage. The series includes The Superior Person's Book of Words, The Superior Person's Second Book of Weird & Wondrous Words, The Superior Person's Third Book of Well-Bred Words, and The Superior Person's Field Guide to Deceitful, Deceptive, & Downright Dangerous Language. Additionally, I would strongly recommend The Gilded Tongue by Rod L. Evans, Ph.D.