Try reading challenging books (I remember Pearl S. Buck’s Good Earth and other classics) and looking up words you don’t know. Having context helps you remember the words. Also, you can make flashcards when you look them up—many websites let you make flashcards and then test yourself, adjusting the difficulty for how much you remember each time. Additionally, go to Toastmasters.org meetings; there is almost always a “word of the day” to learn and practice using. Finally, listen to NPR and spend time with people with a good vocabulary. If you learn a new word, make a point to use it that day. It might also encourage everyone around you to use more of their vocabulary.