It's true: the more you use your brain (and for us writers, the more we use that all-important, creative right brain), the better it will function.
Writing is exercise, and don't let anyone tell you different. Exercising your brain is just as important as working out your body. Getting into the habit of exercising its creative side can be beneficial when it's time to crank out that fiction project.
How, you ask? Well, one of the fastest ways to get your creative gears turning is a writing prompt. In fact, my book, WATER, started out as a writing prompt. It began as a one-page workshop, and we were given the task of explaining, in one page, what might happen if someone entered his or her hotel room to find a stranger in their bed. From that springboard, my story evolved into someone in the hero's closet, caught in the act of rummaging through his things. From there, it was simply a chain reaction of finding out why my heroine was going through the hero's belongings, what he would do about it, and how she would defend her actions.
A simple prompt can be an excellent jumping-off point for a writing project. While I would encourage you to aim for making that prompt a part of your overall writing goal (in my case, a romance novel), it isn't absolutely necessary. A writing prompt is merely a small nudge to get you putting words down. The more you do it, the more your right brain will get into the habit of having those words ready for you when needed. Instead of muscle-building, you're brain-building - teaching that gray matter to flex and do some creative heavy lifting. As with any exercise, the more you do, the easier it will get.
The Internet abounds with writing prompts, but for argument's sake, I will give you one, and challenge you to write one page on the subject.
Your hero is walking his dog on the beach, which he does every night before going home to his big, empty house. As he and the dog are finishing up their walk, he notices a man and woman having a heated argument. Inexplicably, the normally-docile dog jerks its leash out of his hand and bounds toward the arguing couple, barking loudly.
There you have it. Your job is to answer the questions posed by the writing prompt. What does the hero do? Why has the dog's behavior changed? Who are the arguing people?
Questions make the best writing prompts. Have fun, fellow writers, and write on!