I recently came across an interesting quote by singer Tom Waits, who was talking about his 1983 album, a departure from his previous work on which he played mostly piano and guitar.
"Your hands are like dogs, going to the same places they've been. You have to be careful when playing is no longer in the mind but in the fingers, going to happy places. You have to break them of their habits or you don't explore; you only play what is confident and pleasing. I'm learning to break those habits by playing instruments I know absolutely nothing about, like a bassoon or a waterphone."
I think we need to take the same approach with our writing, exercising our minds by trying new things. Maybe our experiments aren't meant for any eyes but our own, but by moving away from the familiar, we prevent our writing from becoming too comfortable, from always "going to the same places."
Try writing a few paragraphs in a totally new style. Maybe write a make-believe long lost page or story by a favorite author whose writing is nothing like yours. If a certain style of writing makes you uncomfortable when you read it, try writing it. Write something that makes no sense but exercises your mind by exploring new directions. Play with words. Write a poem. Write something you wouldn't want your religious leaders to discover, even if you destroy it immediately afterward. Something from a point of view opposite of your own. Anything that gets you out of your usual patterns, no matter how difficult it is or how bad it turns out will help you grow.
If you remember those years when most of your physical growth took place, you'll probably remember them as uncomfortable. Your joints hurt. You were gangly and awkward and looked a little silly. Your clothes never fit right because you outgrew them somewhere between the cash register and the store exit. To grow artistically, you need to go through the same discomfort.
Even if the work you want to publish is firmly within a single genre or style, you want to improve, to grow as an artist and storyteller. If you play around in the bogs of discomfort, even if only in short practice pieces, you will keep your writing sharp and fresh, and avoid getting into the rut of formulaic predictability.