The greatest challenge an author faces when preparing to write is choosing a worthwhile subject for their work.
This may seem trivial at the moment of conceiving, but having a faulty premise kills works-in-progress in their tracks and manifests in various problems difficult to track back to their source.
I don’t have an orthodox method for selecting a subject in mind: if you’re a plot-focused writer or a character-focused writer (Jeff Gerke writes about this in his Plot Versus Character), you will start from a different point. I often work around a theme or motif and take some time to figure out the subject once I’ve developed the message, but I’m cerebral in my approach to stories because of my background as a teacher.
Ultimately, choosing a subject will come down to the personality, proclivity, and aptitude of the writer. However, you can rule out poor subjects by applying a simple test.