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.