Q: Does anyone have any tips on writing sub plots?
Use the character arc as the sub-plot. It will enhance the story if done well. What makes you 'you' are the things that have happened (good and bad) to you in your past and the way in which you chose to deal with them. It’s the same with creating a character and the better you understand the character, the better your story will be.
Where that inner demon comes from is backstory. Every character needs to have a history, but his critical history is short. It's made of the things that changed him for better or worse. How the character overcomes his demons is character arc.
The hero always has something to learn at the beginning of the story. They act out or hide shortcomings and faults that are constantly separating them from what they need and want to achieve, and chances are those are the consequences of having some inner demon that influences their decisions and actions. We do the same in real life, don't we? We try to present our best self to the outside world, hiding our flaws as long as possible.
Remember, readers love a vulnerable hero who realizes his own weaknesses and temptations, and conquers them. That, we can empathize with. We can get behind such a hero and root for him. Because we’ve all been there, we’re all human.
In a character's development he or she might have to overcome:
lack of courage or inner doubts
addictions
lack of ethics
learning to love
guilt
trauma from the past
errors in thinking, etc.
Weaknesses, imperfections, quirks and vices make a character more real and appealing. They humanize them so that the audience can identify with them. Flaws and imperfections give a character somewhere to go, something to work toward. The development of a character is only interesting if they overcome something--the harder it is to overcome, the better the story. Give them conflict and lots of up and downs.
The main plot gives the character an outer problem to resolve, and it should be compelling. It might be the most difficult thing he’s ever come up against and the stakes will be high. He will lose everything he cares about if he fails. Someone is depending on him.
The battle within is what drives the character arc and becomes the sub-plot. Changing is hard. Make it harder by throwing everything you can in his already rocky path. Make him earn the empathy of readers, and deserve whatever resolution he receives in the end.
If the sub-plot focuses on the flaw that wants to prevent him from winning the battle with the antagonist in the main plot, the story can only move forward. By keeping focused--blocks, sequence, and delays in ideas should smooth out for you, and writing will be more satisfying--and fun.