Reader's Digest used to have a pretty good monthly vocabulary quiz in it. If someone did those diligently, vocabulary would rise. Otherwise just read higher level stuff like philosophy, history or classical literature would do it.
You could subscribe to a vocab-building site, like Word-a-Day. Get a study guide for the SAT or GRE. These are designed specifically for vocab-building. Browse your library or bookstore for books of obscure words, troublesome words, etc. Bill Bryson and William Safire are both excellent.
Magazines with celebrated authors are also good - New Yorker, Atlantic Monthly, Esquire. John Updike loves words.
Actually I wouldn't recommend just reading dictionaries or spending your coffee break with a thesaurus cos then it'd just be a case of memorizing stuff and it's be so dull and boring you'd not only quickly lose interest but also retain less than you'd like to.
Experience tells me that learning new words by coming across them repeatedly in context is much more effective. So there's reading more, true, but you've also gotta use the words to keep them in there, to turn passive into active vocabulary - words you know into words you use. You remember the ones you use much more.
Spending time around people with bigger vocabularies means you can freely use your 'big words' without being accused of doing it to sound clever or impress people or make them feel stupid (as people with big vocabs often get accused of doing). It also means you learn more words from them and they from you.
Failing that, try watching documentaries or movies that are more sorta high-brow. Together with the reading that should provide enough input both visually and aurally. But really the biggest leaps are made when you can use the words yourself in both writing and speech.
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