Read Voraciously
Make Friends with the Dictionary: Merriam-Webster's Online Dictionary, Thinkmap Visual Thesaurus
Use It or Lose It
Learn One New Word a Day
Understand the True Meaning of Words: First, you can look at the word's etymology, or the history of its origins. We can add a group or letters in front of (prefix) or after (suffix) the basic part of a word (root) to create new words. About half of English words come from Greek and Latin, so great benefits come from being familiar with these roots, prefixes, and suffixes. There's one more way to make learning and remembering new words much easier. This is when you understand the logic behind the word. Let's look at some examples of how you can see this logic. Notice the two parts of the word "breakfast" to easily see its meaning "interrupt the night's fast" (break+fast).
Maintain a Personal Lexicon: A good option is a computer spreadsheet. This has useful features such as searching, sorting, and filtering.
Follow a Process Vocabulary improvement should be a permanent habit in your everyday life.
Play and Have Fun Merriam Webster's Word Games & Quizzes, Merriam Webster's Daily Crossword.