Ceilings in old houses are almost never flat or level. Often, a ceiling stripped of lath and plaster has joists of different widths or is out of level because of walls that have settled and since been reinforced but not raised. A ceiling 3 in. or 4 in. out of level is not uncommon in older houses.
The leveling method described here results in a perfectly flat ceiling but does nothing to reinforce the structure of the existing ceiling. Where a ceiling is sagging due to major structural problems such as rot, carelessly notched beams, or a sinking foundation, I choose a different approach, which there isn't room to address here.
With just one helper, I can fix even the most crooked ceiling in a few hours simply by sistering steel studs to the ceiling joists. It is always worth the time to flatten out a ceiling. I've found that the few hours I spend flattening can save days of aggravation leveling humps with thick coats of joint compound. A flat ceiling makes for a cleaner and easier crown-molding or cabinet installation. My method also is useful when blending an old ceiling to the ceiling of an addition.