Words are your friends. Having a word arsenal at your disposal makes writing easier, more enjoyable, and spicier. There's nothing like knowing the word that aptly describes what you want. How do you build a solid vocabulary?
The Studied Approach
You might pick a word from the dictionary or elsewhere and then make a point of using it in speech and writing. Or keep a word-a-day calendar on your desk (they also make fun calendars that feature out-of-use words). But you must make an effort to use the words to retain them.
The Casual Approach
When someone uses a word you don't know, look it up to learn the exact meaning(s) and then start using it yourself. Then there's reading. I learn most of my new words this way. When you come across something you don't know, look it up and then use it yourself.
You may even want to add words to your notebook as you go, especially words you love. Or make a note of words you want to look up later as you read or hear them in everday life. A few of my favorite words (earth shattering or not) that I draw on in life and writing are nonplussed, pontificate, hankering, expostulate, and heinous.
What are your favorites?
Over the next several days while reading or listening, pay attention for words you don't know or rarely use. Look them up and conscientiously use them in your own speech and writing. Alternatively, choose interesting or descriptive words from the dictionary and make use of them. Keep a log of these new words in your notebook. You never know when it might come in handy.