One of the intricacies of hanging a door correctly is measuring and cutting the jamb legs so that the head jamb is level. Long-time Fine Homebuilding contributor Jim Chestnut has developed a technique—with the help of a site-built jig—for doing just that. And, he's cut down the process to about 10 seconds. If you have to hang a houseful of doors, you just can't beat that.
To get more door-hanging advice from Jim, read Jigs for Jambs from Fine Homebuilding issue #201 (Feb/March 2009), pp. 106-108.