First of all, what is a subplot? Well, it's a mini storyline that runs alongside the main plot. While it shouldn't outshine the main story, a subplot is a story in itself and serves to reveal more about the protagonist. It can add depth and realism to characters. Subplots are generally introduced early in the book and they're resolved before the climax of the main plot.
I'll use Christine Wells's historical debut, Scandal's Daughter, as an example of expert subplot wrangling. At the heart of the novel is the main plot: a romance between Gemma, whose mother has a reputation to rival certain Hollywood starlets, and her childhood friend, Sebastian, an earl who's allergic to marriage. Gemma would like nothing more than to take charge of her beloved grandfather's estate. Unfortunately, it's not a task for a woman (It's the year 1814, you see.) Gemma doesn't know it, but grandad Hugo is dying and he wishes to see her to marry well. He believes Sebastian is The One for Gemma. Sebastian, meanwhile, has sworn off marriage as he knows it would've pleased his dead but detested father. So he promises to instead find a good match for Gemma. But as the story unfolds, it seems the only man fit to marry Gemma is Sebastian himself, an option neither wants to consider at first. Gemma craves a peaceful, rural life, and hedonistic Sebastian doesn't fit into that paradigm.
One of the subplots tracks Gemma's awkward relationship with her notorious but misunderstood mother. Rumours and innuendo underpin Gemma's shaky position in society. Another subplot is the on-again/off-again courtship between Sebastian's sister and her beau. This was handled in a fun and flirtatious manner that contrasted with Gemma's own romantic woes. There are a few more subplots that provide additional conflict/complications, each with a purpose and each intersecting with either Gemma or Sebastian, or both. By the closing chapters, you'll see how seemingly unrelated details in the subplots gradually lock together and go toward solving the main conflict.
A few points to consider when developing your subplot/s:
* Secondary characters are as important as main characters (MCs). Explore each character's goals, motivation and conflict. You don't have to incorporate all of these details in the manuscript, but it helps to know your characters well.
* Subplots can involve the MC and a secondary character, or they can be about two secondary characters. But find a way for the subplots to intersect.
* If the main story is a romance, you might like to take on a subplot involving the heroine's career/family/greatest fear, etc, etc.
* Don't allow the subplot to overtake the main plot. If the secondary plot is elbowing the main one off the stage, maybe you need to rethink the book's direction.
* Desperately seeking a connection? Find a theme or motif that carries through all the plotlines. Jenny Crusie and Bob Mayer have more on this here.
* Donald Maass says the subplot shouldn't have the same storyline as the main plot (MP). Others emphasise it should still relate to the MP.
* How many subplots can a book have? If it's a short category romance, you probably don't have room to have for anything other than the main conflict. Longer novels can have four or five or more, but I really don't have a firm figure on that. Just don't go for the world record for greatest number of subplots and you should be fine. If in doubt, cut the subplot if it doesn't serve a purpose to the MP.
* A subplot can show a different side to your characters. You might have a hero whose steely gaze alone could cut down a sequoia, but that same man turns into a softy who never misses his niece's ballet recital.
* By the same token, a subplot can be used to add interest or to clarify the main story.
* A subplot, like a MP, has a beginning, middle and end. Its turning points can affect those of the MP. The difference is subplots have shorter story arcs.
Am I repeating myself? Okay, it's time to go, then.