Guidelines for Writing Subplots

I'm hammering out the broad shape of two big subplots in my new book. Sadly, I don't believe that any book will help me with this. Subplots are unique to each story. But some elements are standard for subplots that span the length of most the book.

• Introduction. Characters may be set up in the Introduction or they may first appear in the Early Middle. Since major subplots typically do not start in the Introduction (common exception: major romance subplot), the characters and/or situation are often in stasis at this point, maybe feeling the pressures of the situation (or maybe not) but not yet pushed into doing anything about it.
• Arc. Subplots are structured very like the main plot with a set-up, inciting incident, crisis that makes the character(s) turn from reactive to proactive, a point that propels them into the final showdown, the show-down, and resolution (optional). These points may or may not occur at the same time as in the main plot.
• Timing. Subplots alternate with the main story. Long-term subplot are usually introduced in order of importance (the most important – like a romance – first). And finish up the least important subplots first. Subplots typically are resolved before the final battle of the main plot. A possible exception can be when the outcome of the subplot depends on the final battle's result.