AH! There are so many ways to increase your vocabulary - both in understanding and usage.
A few include:
Keep a log/list of words you don't know, look them up, record the definition - then! USE that word!
*Using words/ phrases puts them into your memory banks.
Read, read, read - then Use the words you like.
Discover the wonders of the thesaurus. If you always use the same word, go to a thesaurus and find alternative words and then USE them.
Find a word that you like, but maybe don't use, and challenge yourself to USE that word three times a day for the next few days.
Are you seeing a pattern? USE a new word to make it yours, to own it.
These tips should get you started.