Open a dictionary at the beginning of each day. Turn the dictionary to a random page and place your finger on a word. If you already know the word, continue this process until you find a word that you do not know. Learn the word and its definition, and commit to using the word in a sentence several times that day to incorporate it into your regular vocabulary.
Browse to a reference website with a "Word of the Day" feature each day. Dictionary.com, Merriam-Webster Online and OneLook are three online dictionaries with this feature. Depending on the website, the word of the day may be picked randomly, or it might be based on what other people are searching.
Subscribe to an email list that sends subscribers a new word every day. Three services with a "word of the day" email subscription feature are Dictionary.com, Wordsmith.org and YourDictionary.com.