Reading. There are different styles of speech that sound "smart" to different people. If you like academic (especially, statistical/mathematical) jibber-jabber, you would have to read (ideally study) a lot of academic works (textbooks, papers, blog posts, whatever). In time, you'll pick up a lot of that style of speech. If you think the political/financial articles in NYT, Economist and other such publications are the epitome of the sophisticated speech, you should definitely read those papers voraciously. Their jive will rub off on you whether you like it or not. The only problem: if you're not a native speaker of English(I am not, btw), your speech will sound a bit stilted. Anyhow, this is my experience. As a kid I used to prefer spending my time reading 19th century Russian writers (hand-me-downs from older generations) and newspaper articles on current matters to spending time with my peers. My speech definitely reflected that for a long time because people would often comment how weird (and funny) it was.
Personally, the smartest sounding speech to me is the one where people get their point across in fewest words and clearest way possible.
One of the current stylistic techniques(?) that I really dislike is using a long winded quote as a descriptor of something/one.