Choose a new word to add to your students' vocabulary every day. Read the definition, etymology, common uses, part of speech, synonyms and antonyms. You can read these at a variety of websites, such as Cambridge Dictionary Online, or at Thesaurus.com. You can choose words to incorporate into your students' vocabulary from the dictionary, or pick your words from references such as The Academic Word List. After you understand the word and its definition, try to use it throughout the day in your lessons, and encourage the students to use it as well.
Play vocabulary or word games. Choose a word and have students come up with all the synonyms and antonyms. Crossword puzzles are also a good way to increase vocabulary.
Use flashcards. Instruct students to write words they wish to incorporate into their vocabulary on one side. On the other side, they should write the definition, synonyms and antonyms. Encourage the students to go through the flashcards giving the definitions, and then using the word in a sentence.