During my highschool days, the subject of vocabulary had not been enforced or rather most students were lazy about expanding their vocabulary not until in the final year when our school principal asked to see our vocabulary book. I'm putting this question out here: what is the best way one can improve ones vocabulary?
"The best way" is subjective because what is best for me, might not be best for you. Regardless, a generally good way to increase your vocabulary is to be a curious reader.
That means you should not skip over words you don't understand in new books that you read. Instead, you stop, open a dictionary or google the word. when you do that during reading, you suddenly understand the context better than you did before. The story is making more sense now and you are more likely to remember the words you learn while reading.
Read often and invest an effort in searching the words you don't know.
Many e-Readers and tablets have an integrated dictionary as a feature. As you read the book, you can highlight any word and immediately get a definition - which gives you the opportunity to broaden your vocabulary without disrupting your reading experience.
Of course, you might want to choose books that are outside of your comfort zone, such as literary classics with broadened vocabularies compared to more casual reading; another boon is that many of these books are available in the public domain.
When I was young, I developed an appetite for learning new things. I began reading the dictionary (analog, not digital) in my spare time, just to discover new words. As a result, I developed a healthy vocabulary. The key word, as @Oren_C states, is curious. In many ways, this is an antonym to lazy.
If you ever find yourself looking up a word (for definition, or synonyms, whatever the reason) just read all the synonyms. I've found that when I actually click on the arrow provided when you look up a word's definition, there are often a crap-ton of synonyms to see. Even just scanning them quickly constantly shows me words I didn't know existed, and even if I don't use them, I can get a good laugh at some of them or have them on my radar screen for the future.
(Like did you know "hangdog" means having a guilty appearance? Or that "nonplus" is another word for fluster? Or "like a cat on a hot tin roof" is a an accepted way to say something is nervous? I certainly didn't.)