I guess I'm lucky in that my subplots develop as I write the story. I don't often have to put much thought in their creation because they show up among the main story and from the character development. At times with a story idea I'll even know more about the subplots than I will about the main plot until I later have the time to develop the story when I'm almost ready to start writing it. They just come along with the writing for the most part, so guess I'm not much help.

Good luck with yours.