There are a lot of tricks like this in various fields. One of the simplest ones I can think of comes from the visual arts: when you come to a stopping point, simply turn the paper (canvas, Photoshop workspace, etc.) upside down, and if any of it looks weird from the new perspective, fix that part.
Unfortunately that'd only work for the static parts of UI; I don't know of any better palate-cleanser for the interactive parts of the experience than simply setting the project down for a while and coming back once you've context-switched into something else.
Perhaps you could try randomly changing parts that aren't what you're focusing on, like drastically changing the color scheme.
The guy who taught me how to eat sushi a couple months ago explained that I should get a slice of ginger before trying a different kind of sushi in order to "reset my taste buds" (probably broscience) and feel the taste anew (it works). That's also the exact problem that programmers face when trying to design user interfaces: after looking at the thing for a long time, they grow blind to its shortcomings. A visual reset would come in handy. I imagine that resets could help in other areas too. Does anything like that exist? Cogsci majors, help!