Read books, papers, etc oriented towards a more literate audience. There's a massive difference in vocabulary between a Harlequin romance novel and Hayek. The goal isn't to learn a handful of uncommon words to add a layer of pretentiousness to your vocabulary, it's to learn the most accurate way to express yourself. See Spot run is vague, while observe Spot circumambulate his centenarian master's ramshackle dwelling with hackles raised is far more precise. You need to see and use your vocabularly routinely to truly expand it to the point that it becomes natural.