Just read.
I grew up with a lot of British writing, so I adapted that style for some time. I've had people separately tell me I write like C.S. Lewis--if that's the case at all (I'm not so conceited to think I'm half as clear, expressive, or eloquent as he), it's because I consumed his books since fifth grade, and could spit out whole lines of dialog on command at one point.
Reading helps you contextualize language and absorb both the vocabulary and syntax. It teaches you how to speak properly. This makes it important to read the right kind of material, of course, and not just journalism (which is often inherently manipulative) or blog posts. Fortunately, my parents kept me away from the newspaper when I was younger, so I could get lost in those silly fairy stories that are now widely considered kitsch and cliche. Those fairy stories prepared me for my career and taught me how to write and speak.
I must concede I'm not an eloquent speaker, but in the right mindset--that is, talking like I am putting words to a page--I can draw from my more extensive writing vocabulary.
I know this thread is specifically about vocabulary and I didn't address that, but for me, it all goes hand-in-hand. I never expanded my vocabulary without also expanding my knowledge of how to use it and how to clarify my language. And of course, I was always one to absorb the overall style of any book or article I read, so my vocabulary is a direct result of my literary interests.