No easy fix, if the problem is in the roof buildup itself, unless you're talking about just a half-inch or so of "dishing" over a short run. You could eliminate that by building up with a heavy course or two of mineral granules application over liquid asphalt.
If that doesn't appear to be the case, and the sag is more significant, you might want to pull a section of sheetrock to determine the size and spacing of joists supporting the roof. An engineer could tell you if your joists are undersized for the span, and possibly suggest viable solutions. The installation of flitch plates comes to mind, glued and screwed to the bottoms of the joists while they are in a pre-cambered condition. It wouldn't be cheap, and would require redoing the entire ceiling under the roof.