For me, some parts of writing come much harder than others. Dialog, for instance, comes very easily, while subplots are the bane of my existence.
I overcome my trouble with subplots by looking over my outline and asking myself, "What else could go wrong?" for every scene. The best of the "what-elses" are the ones that add the most tension, have the most far reaching consequences, and tie back into the main plot most successfully. Those are the ones that make it into the rough draft.
As simple as it sounds, it took a lot of hair pulling to come up with that system and subplots still don't come easily to me.
Reader Question: What are your strengths and weaknesses? What ways have you found to overcome or work around your weaknesses?