Gamification
That doesn't mean that learning these new words has to be a stuffy affair. Far from it - in fact, advances in technology have made learning new vocabulary very easy, if not fun. This is because of the high levels of gamification or turning memorization or mundane tasks into games. Many Internet sites and mobile apps offer a variety of games to help you learn vocabulary. These may not be as engaging as the newest console best-seller, but at least they'll make the process less painful. Several of these will even permit you to upload your own lists, so you can target specific terms of interest.
Back to the Roots
Whether by reading or playing games, you'll eventually notice that many words repeat the same letters. Look at a list of sciences and you'll see many of them end in '-ology.' This is because '-ology' is a root, or fragment of a long-forgotten word that now makes up a part of the word. Many of our roots come from Greek, Latin, or French. Being able to know these roots will help expand your vocabulary. For example, if you knew that 'anthro-' refers to 'humans,' while 'morph' means 'shape,' you could know that anthropomorphic means having a human shape. In the rare event that you ever have to describe an alien, using a term like 'anthropomorphic' will make you sound slightly less crazy than 'looked like a human.'
Other Languages
If you've ever read much about the great minds of generations past, it seems they all had the same education. In addition to learning how to write in English, they studied some combination of French, German, Greek, and Latin. For those of us looking to expand our own vocabularies, we would do well to learn a few basic words in these languages. Around 60% of everyday English comes from French, which itself was heavily influenced by Latin. Much of the remaining 40% comes from German, while Greek makes up much of the technical language.
The examples I gave earlier for anthropomorphic both came from Greek. Later tonight, I'll have beef for dinner, which comes from the French boeuf. The animal that beef comes from is a cow, which is koh in German. It is not through some higher-level study of these languages that much of our vocabulary comes - in fact, each of those words was chosen from the first chapter, if not the first pages, of books to study each language.