The typical definition of theme is usually something like this:
A theme is the central idea or ideas explored by a literary work.
The problem with this type of definition is that it's not very specific or helpful when coming up with the theme of your work. It leads to weakly stating themes as concepts or ideas like "death", "justice" or "love". Stating your theme in this way is just too vague to be of use.
So what about themes such as "all men are liars", or "people are basically good"? This is a step in the right direction, and certainly more helpful. The problem with these kinds of statements as themes is that they are really just opinions. They lack power.
Take A Stand
An alternate way of looking at theme, which I first encountered in the wonderful book Creativity Rules! by John Vorhaus, is defined in this way:
An instruction, strongly expressed as an imperative, is the story's theme.
No one would argue that great, memorable stories have powerful themes. And powerful themes are instructions. They include a call to action. Using an exclamation point is not out of the question when stating yours.
What separates a powerful theme from the concepts and opinions above is that it takes a stand. A great theme tells people how to act and does so with authority: Embrace change! Be Strong! Destroy Evil!
Taking a stand thematically makes your writing clearer and more powerful. Once you really know what you want to say, it also makes writing easier.
Remember that writing is an seemingly endless chain of decision-making. If you are clear about what your theme is then making those decisions is a lot less difficult. Anytime you have a choice to make, choose the option that supports your theme and you'll be building a powerful, cohesive narrative.
Won't This be Too Preachy?
Just because you are choosing a powerful, instructive theme does mean you are preaching. Having a well-defined theme that takes a stand does not mean you are writing a polemic, or "hiding" a moral message in your fiction.
Your theme does not have to be profound to be clear and powerful. It does not have to reflect a "higher" purpose or try to fix the world's problems.
Consider a fun, entertaining comedy film like Home Alone. It's theme is "Protect your home!" It's crystal clear, powerful and takes a stand without being preachy at all. There are countless enjoyable thrillers with the theme "Seek Vengeance!", which certainly isn't a lofty moral message.
Can the Theme Change?
Also remember that the theme you choose to start with isn't always what you'll end up with. If it changes organically as your story moves ahead, then feel free to adjust it. But by starting with a powerful theme in mind, you'll have a default to fall back on if necessary.
One way of coming up with a theme at the start of a writing project comes from John Vorhaus. He suggests asking yourself, "If you could teach one other person in the world one thing, what would that one thing be?" The answer to that question is your theme.
Choosing Not to Decide
Remember that whether you consciously choose a theme or not, your stories will have one. It may be weak, it may not even be what you consider the point of your story, but it will be there. And readers will pick up on it.
Although the details of your story will fade over time in your reader's minds, the theme stays with people. So actively choose the theme you want to leave with your readers rather than leaving it to chance.
So next time you're stuck thinking of a story idea, try starting with a powerful, instructive theme and see what happens. You might be surprised.