Mark the location of the trusses behind the entire ceiling in the room, using a stud finder. If you have trouble pinpointing the location of the trusses where the ceiling sags, send an assistant into the attic to help.
Cut 1-by-3 wood strips that span the width of the room, in the same direction as the trusses. You may need to use multiple strips to cover from one side of the room to the other, depending on the size of the room.
Drive drywall screws through the wood strips and into the trusses above the ceiling, holding the wood strips firmly in place. The screws need to be spaced out about every 2 feet to ensure the wood strips do not sag later.
Attach new drywall sheets to the wood strips, driving drywall screws through the perimeter of the drywall and into the wood strips. Spread drywall compound into the joints between the drywall sheets, using a putty knife. Apply paper tape to the joints, over the compound, and wait for the compound to dry.
Spread one to two more coats of compound onto the paper tape, until it sits flush with the drywall. Sand the compound with fine-grit sandpaper, once the compound has completely dried. Roll primer and then paint onto the drywall ceiling, allowing the primer to dry first.