Write down every word you hear or see each day that you don't know the meaning of. At the end of the day, look up the meaning of the word in the dictionary.
Flip to a random page of the dictionary each day. Choose a word on that page that you don't know, then memorize the word and as its meaning. Make flashcards with vocabulary words on the front of the cards and the meanings of the words on the back of the cards.
Substitute your newly learned vocabulary words for old ones in everyday conversations. If there is not a way to naturally work the words into conversation, then write down sentences that use the new words in context.
Look through any writing you do, and look for a pattern of overused words. Substitute those overused words for new ones. You can do this by flipping to the overused word in the dictionary. Next to the meaning of the word, it will list synonyms, which are different words with the same meanings.