Structural designer Hank Graftenreed contributed an AutoCAD tip that lets you rotate your UCS very quickly and precisely.

"Draw or select a line along the longest side of the property, building, etc. (I typically use a line on the southwest part of the drawing). Type UCS at the Command line, type OB for object, and click near the left end of the line to establish it as X. Y will be up and perpendicular to the line. (Note: Picking the right end of the line will result in Y being down instead, which may be desired.) Type Plan at the Command line, type C for current, and press Enter. If current is the default, simply press Enter twice.
 
"This can also be done in a viewport without affecting model space, but you will need to reset the VP scale."

Notes from Cadalyst Tip Patrol: Rotating your UCS to an object is a very quick way to rotate your drawing's view so that it is aligned with a major object in your drawing. This works very well when you want to rotate the view in a viewport. It is instant and it rotates the view to the angle of the object you select. To put things back to normal, type Plan on the Command line and choose World, then type UCS and select World.