Here’s an interesting article by Allen Palmer, an Australian screenwriter who’s worked in Hollywood and the UK. His blog, Cracking Yarns, should be on the go-to list for any writer in my humble opinion.
When I write, I’ve got an idea in my head best described as: this is my character, this is the problem, here’s how he’s going to solve it, and here’s what happens once he’s solved it. Putting my project head on, that’s the critical path.
Woven in between that are sub-plots: the stuff that makes the protagonist’s journey interesting to readers (and me as the writer): what goes wrong; who are his adversaries; who are his ‘helpers’, what are the barriers to his success, etc.
Allen’s written a great article on how to make your sub-plots work. It’s entertaining, brutally honest, and available here.