Read as much as possible. If you read, you are likely to come into contact with many new words, all of which will help you. Reading books from different historical eras, or books written in a different style of prose, will help you to encounter new words. For example, reading "Clockwork Orange", "On the Road" or "Pride and Prejudice" will all bring you into contact with new words and language.
Write new words down. Use a journal or diary to keep a note of words you learn or read. Looking up the official meaning in a dictionary when you write it down will help you to use the word correctly. Alternatively, look the word up in a thesaurus and write the most commonly used synonym. For example, write the word "hubris" and then write in parentheses "arrogance/self-importance."
Buy a "Word a Day" calendar, or sign up for a similar newsletter. This way you will be forced to engage with a new word everyday. These words are usually deliberately obscure, meaning that the chances of you coming across a word you already know will decrease. Write these down in your diary to help you remember, and try and use the new word throughout the course of the day.
Learn a language to help you form links between words in English and their etymology. Some of the best languages for understanding English better are French, Spanish or Latin. Learning words in these languages will help you to remember words in English. For example, learning that "felicity" means happiness is easier when you know that "Feliz" means "happy" in Spanish.