There is no substitute for reading. This is especially true for children who are developing their vocabularies. But adults can increase their vocabularies as well by reading and looking up words they do not know. It may make reading slower at first, but in time will increase reading speed and word comprehension. Remember to keep a dictionary handy while reading and take the time to look up unfamiliar words.
Vocabulary can be increased by writing any unfamiliar words, along with their definitions, in a journal. Keep referring back to the list until the words become familiar. Journals have the added advantage of creating positive reinforcement by providing a record of words learned.
Keep a record of unfamiliar terms and study the meaning of one word each day. This can add about 30 words to a person's vocabulary every month. This method may be too rigid for some people. If so, try another.
Learning root words will not increase word memorization, but it can help clarify the meanings of unfamiliar terms. Most words in the English language are based on Latin and Greek prefixes, suffixes and root words. Thus leaning a few prefixes, roots and suffixes can help decipher a large number of much longer English words.
Link unfamiliar words with words that have similar meanings. It may be difficult to remember the definition of a word such as "superfluous." So remember that it has a meaning similar to the words "extra," "redundant" and "unnecessary."
Some people think more visually than others. These people may want to associate a new word with a visual object. When memorizing a new word, think of what the object looks like. Of course, this works best with words that have concrete examples. It is much easier to use imagery to memorize "computer" than "felicity."