Read as many books as possible. Don't just stick with one genre either. If you like romance, choose some adventure type novels, or if you solely read graphic novel, pick a Pulitzer prize winner to read. Vary your reading selection to get a broad base of vocabulary words.
Get a notebook and designate it only for building your vocabulary. List any word you come across in your reading materials and write in the book. Look up the meanings of the words and list a definition next to the word you wrote. Try to use that word within your conversations and discussions throughout the day.
Review your vocabulary with flashcards. Cut out squares of card stock or index cards and punch a hole in the top center of each one. Write an unknown or uncommon word on one side, and list the definition on the other. Slide the cards through a metal ring to keep them together and close at hand. Before writing a paper or taking a test, review the words to use them in your projects.
Check out GRE or SAT tests. Older tests from prep materials for GRE or SAT tests will have an abundance of higher level words. Use words from these lists to boost your vocabulary to higher levels.
Increase the depth of your reading material. After you have expanded the selection of the types of books you read, increase the complexity of the materials. Read books on classic philosophy like "Answering the Question: What is Enlightenment?" by Immanuel Kant, or science materials such as "A Brief History of Time" by Stephen Hawking. Higher level reading materials will help you expand your vocabulary.
Look up the prefixes. Learn about the origin of your new-found words and discover the heritage behind roots, prefixes and more. Many words in the English language come from Greek and Latin words. Get an idea of the real meaning of words by learning where they come from.