Honestly, as counter-intuitive as it seems, sometimes the best thing you can do is step away from whatever you're working on for a break and come back with a fresh mind and set of eyes.
The number of times where I've done that and returned to whatever I was doing and just "...Oh. Duh" after returning... Always after the wall has thoroughly tenderized my head and I throw in the towel because I never learned my lesson, of course