Leaving a project overnight in order to gain fresh perspective the following morning.
Described in detail at The San Francisco Egotist
And this from Chuck Wendig in 25 Ways To Unfuck Your Story:
LET IT SIT AND PICKLE
Writers need time away from their work. Go at it too soon and you either hate it too much to let it live or love it too much to cut it with your steely knives. You need enough distance from the work to let you read it and believe that someone else wrote it — that distance allows you the cold, dispassionate dissecting the tale needs. Maybe that means you leave it for two weeks, two months, or two years. That’s on you to figure out. But when you dig back in, you’ll be amazed at the clarity a little time has afforded you. The trouble spots will start to stand out like a shadow on an X-Ray.
And this from Zadie Smith at Wired:
When you finish your novel, if money is not a desperate priority, if you do not need to sell it at once or be published that very second — put it in a drawer. For as long as you can manage. A year or more is ideal — but even three months will do…. You need a certain head on your shoulders to edit a novel, and it’s not the head of a writer in the thick of it, nor the head of a professional editor who’s read it in twelve different versions.