SIGN UP FOR ANU GARG'S "Word.A.Day" e-mail list! It's fantastic... great definitions, always a couple of examples of the word in use... massive archives in case you want to look at more... sometimes they'll have themes for the week, like "portmanteau words" or "Ancient Greek"... even if you
do know the word, their breakdown of the etymology will often furnish you with a lot of new information that adds to your understanding of a word which you thought you knew a lot about before... for instance, where do "juggernaut", "taboo", and "hocus pocus" come from? (they're all from completely different sources)
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Read... okay... read... but read what?!?!??!! You may not be into certain 19th-century authors... so there are others who'll expand your vocabulary while making you interested in finding out exactly what the hell they're talking about when they use words like "marmoreal" or "nacreous" or "nictitating".
For a great 20th-century novel, read William Styron's "Sophie's Choice"... high-brow vocabulary... and while being literary he's also very hardcore... it's just a well-written book, worth reading entirely for its own sake.
There are others but I sincerely doubt you're going to run to a bookstore and read all ten of my recommendations, so I'll leave you with Sophie's Choice...
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Oh, it should be obvious, but when you read, you should be noting these words down (either in a separate exercise book or on some of the blank pages at the end of the book you're reading)... if you're into the story and don't want to stop every time you need to look a word up (though in the age of Google it hardly takes 20 seconds), then when you write down the word, note the page number too...