Good writers eventually develop a style all their own. As contradictory as this may seem, sometimes one of the best ways to firm up your style is to play around with different things. Take a piece of writing that you’re already working on (or have finished). Pick just one discrete self-contained section of this piece, no bigger than a couple of pages at most (but hopefully at least a couple of paragraphs). Now, re-write this piece several times using different styles of writing. If you normally write non-fiction in a didactic, educational tone, write it playfully. Write it somberly. Write it melodramatically. If you write fiction experiment with melodrama, florid purple prose, dry bare-bones description, action-packed excitement, etc.
If you want a follow-on for this, pull several books off of your shelves (or take them out of the library) by authors that have distinct, recognizable styles. Spend a little time reading a few pages from each book and making notes (mental or physical) as to what that style consists of. Then try to re-write the excerpt from your piece mimicking those styles. While plenty of people will tell you to try distinctive writers such as Lovecraft or Hemingway, I also recommend trying lesser-known writers from your own favorite books to make sure you’re trying something new and different. If you don’t mind spending several days’ worth of exercises on this, try to mimic at least four or five writers’ styles, and make sure they’re very different styles.