We all know about Q in JOSM, which will make the angles in a building 90°. But isn't it annoying when you have one building, you are adding an adjacent building, and when you square the corners it affects the first building? Here's a pro-tip. When squaring the second building, select it, then add 2 nodes from the first building into your selection. Then hit Q. It will use the first building as a baseline for the second. Notice 2 nodes are highlighted red.