Start by choosing a wide variety of literature to read from and consequently lead yourself to explore a broader range of vocabulary. Select works from different eras (classic to postmodern), and different genres (fiction, non-fiction), etc, all of which will contain different kinds of words. In addition to reading literature, make sure you also look into books that revolve around history, science, politics, etc. In short, expand the range of your reading.
Next, in order to make sure that you get plenty of reading time, set yourself a booklist that needs to be finished by a particular deadline, and set a particular time for reading daily. These small measures will motivate you to read regularly, and thus, to explore more vocabulary.
Make yourself a vocabulary notebook. You will need to keep this with you at all times as you read, so that you can jot down difficult words, their meanings, and the sentences in which they were used. There are a number of ways this can be organised – you can either make alphabetical sections for noting down words, or jot them down in groups according to which book you read them in. The latter is likely to prove more stimulating for your memory, and help you improve your vocabulary faster.
Every time you sit down to read, be thorough. Enjoy the work but remain on the lookout for words you might not know the meanings to. Most readers have the tendency to keep reading on, and not bother to look up the meanings of words they might not know. However, every time you encounter an unfamiliar word, focus on it, and try to use contextual clues to guess what it might mean. Then, use a light pencil to underline the difficult word, and move on.
Once you are done reading for the day, go over all that you read, and look for the underlined words. Jot these down in your notebook and look up their meanings in a dictionary. Note the meanings down next to the word, along with the sentence in which the word was used. Once you have written down the difficult words of the day, go over the list again, and study it carefully. You can also consult an online dictionary for the correct pronunciation of the word.
Keep reviewing the words in your notebook as the list grows longer. Try to incorporate these words into your conversation and writing, so that they become a fixed part of your vocabulary.