Read as many books, magazines, newspapers and other publications as possible. Reading words in context with the rest of the sentence is the best way to learn words. It will enable you to see not just the word itself, but also how it might be used in a sentence and what its likely meaning is based on how the author uses it. If you come across an unfamiliar word, stop reading and look it up in a dictionary, which will reinforce it in your brain.
Write each new word you come across in a day on an index card along with its meaning and how it might be used in a sentence. Keep a stack of these handy and practice them for a few minutes every day. Create a new sentence each day that utilizes the words in different ways.
Play word games with friends or by yourself. Play games like Scrabble and Boggle with friends to exercise your own vocabulary, or use it as a chance to pick up on a few words your friends know but you don't. Set aside time to figure out crossword puzzles by yourself as another entertaining way to learn new words.
Check out Dictionary.com's Word of the Day webpage every day. The site features a new word, its pronunciation, part of speech, meanings and even some quotes in which the word is used. Using this feature every day could help you learn as many as 365 new words per year.
Keep a thesaurus handy at all times and make liberal use of it. If you're stuck trying to figure out an effective word to use, reach for the thesaurus to explore alternatives. Or, just explore your curiosity while you're reading and look up other words an author could have used in that situation.