Last May, there was some discussion about sagging doors and out of plumb walls. -Mike- suggested removing one screw on each hinge and replacing it with a long screw that goes through the hinge, shim, jack stud, and king stud. (If there's no shim behind the hinge, you can pull the door out of alignment so it won't shut properly.)
The suggestion works really well, and I put a 3 1/2" screw in an exterior door that was sagging. The previous owners simply barred the door with a couple handles and a metal brace, but the single long screw brought everything up in to alignment again. The bathroom door that I was complaining about in the first place also no longer sags.
Thanks -MIKE- for the tip.
Puckdropper