Read from a variety of genres, both in fiction and non-fiction. Particular styles, narrative mechanics, and themes work differently or not at all depending on what you're writing. Understanding particular tropes and themes will help you to spot when you're relying on them too much, as well as helping you to alter them or change things up if you so desire.
Also, if you can help it, either take classes on literature and writing, or study it when you have free time. Having a grasp of proper grammar, as well as having a sizable vocabularly, allows you greater freedom of expression and a greater ability to affect the kind of tone you want in your story. Not to mention that it helps relay what you want to detail and convey to the reader better.
Finally, I've got two important things to tell you that every writer should commit to memory and fully embrace. First is that "writing is re-writing." The editing process of writing is such an important part of the process. It helps to make sentences and paragraphs more concise while maintaining detail, and it fixes mistakes or breaks in the stream of consciousness that could happen with a reader if they come across a part that is written incorrectly or is hard to digest. This fits into what others have said about just continually writing. Be it term papers, roleplays, poetry, prose, or just blogging, the more you write, the better you get at it. If you are unwilling to edit and rewrite your work, making various drafts as you refine and improve, then you shouldn't bother being a writer at all.
Second is that, if you want to write, you must have something you want to express or convey to the reader. A good writer almost always writes about something that is of interest to them or that they care about, or at the very least they write when they want to say something. That is the purpose of writing, and communication in general. That desire must translate through the pages, otherwise the reader will eventually sense your disinterest/lack of interest, and they'll stop reading. Or even worse, they will think you a terrible writer or a phony, and then stop reading. If you care about something, or want to inform or convey something to the reader, you will take extra care to make it compelling, yet easy to understand for the comprehension level you're writing to.
I embrace these rules and have done the above myself, and I feel that I've become a stronger writer as a result. Taking constructive criticism about your formatting, sentence structure/grammar, and how wordy or how few of words you use is also helpful. Outside of that, everything else is using what you've learned and improving upon yourself.