Listen to people who have superior oral expression. Notice how they use words accurately and precisely. Seek out good speakers on the radio, on news programs or in documentaries. Go to lectures by renowned speakers, or listen to books on tape. You will acquire a more powerful vocabulary, hear words in correct context and learn correct pronunciation.
Set aside time every day for systematic vocabulary study. The authors of "30 Days to a More Powerful Vocabulary" say you can improve with only 15 minutes a day. Study word roots, those Greek or Latin words underlying many English words. Also study word beginnings and endings, which are called prefixes and suffixes. Understanding word formation and relationships between words will build your vocabulary.
Read challenging books in English. Find books that interest you, but make sure the vocabulary makes you stretch. When you find a word you don't know, try to guess the meaning by the context. Also see if it reminds you of a similar word with the same root or base word. Use a dictionary only after you have tried to guess the meaning.
Practice new words on the go. Copy words you want to remember onto note cards. On the back, include the meaning and a complete sentence so that you learn the word in context. Carry a few cards with you and practice when you have the time. Then make a game of using your new words in conversation.
Study groups of words on similar topics at the same time. You will find it easier to learn words together that have related meanings. For example, learn the names of various scientific specialties. Norman Lewis, author of "Word Power Made Easy," uses this method, among others.
Practice crossword puzzles, and play vocabulary board games. If you have friends with excellent vocabularies, include them so you will learn more by their competition. Having fun while you learn new words will help you acquire a power vocabulary.