A friend asked me how best to expand her vocabulary. I told her that she needs to encounter new words and use them. The problem she had, though, was remembering the words long enough to use them.
Humans in general are terrible at knowing things after seeing them once. We have to practice and try over and over again. We need experiences and memories to draw from. That's how we learn. That's how things become natural.
If you want to expand your vocabulary, pick a word you want to use more often. Today's Word of the Day at Dictionary.com is "bespeak". Go and look at the word's definition. See the examples used with it. Get a feel for how it can be used. Go and write 20 sentences using it. Write 20 more sentences after each meal. Slip the word into conversations. See if you can do it so naturally that people don't question it.
If you want to learn a word, use it deliberately. There are tens of thousands of words that you could easily go your entire life never needing. But if you would like to learn a few more than you already know, it can be done and it's not terribly difficult. You know lots of them already. What's a few more?