As a child, did you blow bubbles? If so, you already understand something about subplots.
What does blowing bubbles have to do with subplots, you ask?
Simply this: When you blew too hard on your bubble wand, you burst any bubbles as they formed. Blowing too lightly, while it showed you there were bubbles to be made, didn't produce them. Only by exerting the right amount of force could you blow bubbles.
Subplots are a lot like bubbles. If you try too hard to produce them, they evaporate. However, they won't necessarily form without your help.
What's a Subplot, Anyway?
It's easy to become confused when thinking about subplots, so let's start with a definition. A subplot is secondary plot that compliments your main plot. Adding subplots to your novel will give it layers of substance and effectively underline your theme. Layering with subplots adds texture to a story.
Subplots should never lead the reader away from your theme and should, in fact, support your primary plot. A subplot happens because of (rather than instead of) the main story. Anything else is a distraction otherwise known as a rabbit trail. All sorts of unrelated events tangle together in real life. Good fiction doesn't suffer from such snarls but is carefully constructed to represent, rather than emulate, real life. Understanding this difference is crucial.
Most people would agree that Gone With The Wind details the epic romance of Scarlett O'Hara and Rhett Butler. Scarlett's relationship with Melanie Wilkes, her father's fate, and her relationship with her sister, Sue Ellen, are all subplots. Each forms its own "story within a story," and yet each contributes to the greater story by shaping our opinions about Scarlett. None of these subplots is forced. Each arises naturally from the main plot and helps develop the theme.
One of the best ways to add subplots to the main storyline is to introduce new scenes from the point-of-view of the characters involved in them. This is a great way to introduce secondary characters, by the way.
Remember, unless you are writing from an omniscient viewpoint, never change viewpoints within a scene. Provide either a scene or chapter break whenever you change point-of-view. Using other characters' viewpoints to unfold subplots means you can introduce information to which your main character is not privy. Just remember, as you weave your storylines, to connect them at the end of the book. Don't leave any threads hanging.
Do you have a hard time coming up with subplots or do they occur to you naturally?