Subplots are a tricky issue sometimes. Without them, your plot can come off stale, impersonal, simplistic, and boring. In fact, without subplot, there really can’t be any character development (unless the resolution of an internal conflict is your main plot). Too many subplots, and you can spread yourself too thin, confuse the reader, get lost in tangents, and generally make a mess of things.
Paying attention to what works for me as a reader, I’ve decided that the best subplots are the ones which play off of the main storyline. Preferably, a subplot not only stems from the main events of the book, but also, in return, affects the main storyline. A sort of feedback loop of cause and effect, each building off of one another. Get a few subplots like that going at once, and your story will practically write itself (and everyone will think you’re brilliant for pulling it off (not that I’ve experienced that part as a writer, just noticed as a reader which books I find brilliantly put together)).
George Elliot and Terry Pratchett (who probably never would’ve expected to be compared within the same sentence) are both masters of interweaving an overall plot with smaller storylines.
The last book I wrote was so narrowly focused (intentionally so) that in the rough draft, I left out all subplot, just making notes to myself of subplots that occurred to me. Anything that didn’t hold together or any characters that weren’t coming across as full, rounded-out people, I worked through in the second draft by stirring in a few of those back burner ideas from my notes, and that’s how I knew what subplots were actually needed to carry the story off.
I won’t be so lucky with my NaNoWriMo novel in November. It’s a huge storyline with multiple conflicts playing off one another and a cast of thousands–no, I exaggerate…only hundreds…er…well, dozens, anyway. And all those characters have their own issues and their own parts to play, and things to overcome that will affect everybody else. It’s rife with subplots and potential for more to pop up as I go along, and frankly, I’m a little intimidated by that. But I’ll take a page out of my own book (haha, I make funny) and in the rough draft use only what I know I need, making extensive notes for things I’m not sure about. I’ll let you know how it goes. Hah! Wish me luck!!