Answer:
One of the most common and effective ways is to read as many books as you can, and to keep a vocabulary journal.
Whenever you come across a word you don't know the meaning of, don't just look up the definition in a dictionary; write down the meaning of the word in your vocabulary journal, as well as a use in a sentence for the word. Doing this forces your brain to remember the definition of the word at a level deeper than if you had only looked up the word's meaning.
Hope this helps!
