Before I went to law school I was a secretary for the Utah Capitol Preservation Board. The Board oversaw the work done by the Architect of the Capitol and his staff during the Capitol’s renovation. One of the women I worked for was the Curator of the Capitol, who managed all of the art in the Capitol’s care and oversaw the restoration of the painting of the Capitol dome. (Walking the scaffolding to the top of that dome was one of the scariest things I’ve ever done but so amazing.) One of the tricks she taught me was how to ensure the state’s wall art stayed straight after hanging.
Once you have the art straight, apply a small circle of velcro on a corner of the back of the frame. Apply the velcro so that one sticky side is on the back of the art, the hooks and loops are stuck to each other, and the other sticky side is ready to stick to the wall. Making sure that you’re perfectly level, push the art against the wall.
Voila! Perfectly straight art work.