The most obvious – and easiest! – way to gain perspective is to put your work away for a while.
The truth is, we don’t know how taking a break frees up the mind, but it does: Somehow it freshens our little neurons, or perhaps it prompts the brain to create more cleverness molecules.
If you can bear to let a short piece sit a week and a book-length work a month, do so. Longer is fine, too; some authors have abandoned manuscripts for years before unearthing them and realizing, ‘Hey, this isn’t bad,’ and renewing their energy for the project.
Subplots bring realism to your main plot simply by existing – by interrupting the flow. Why is this? Because life doesn’t move forward all at once. Interruptions happen, change rushes in, we juggle three or ten balls at once. Readers don’t expect continuous narratives.
I once had an editor advise me, as I was revising one of my early novels, to add more characters. I played around with the idea. As soon as I'd decided a few fresh faces and give them something to do, I realized that what my editor had really asked for was more plot. Ding. More characters equals more action.