Aim a laser level across the ceiling grid to locate the lowest point. Set up a ladder underneath the ceiling grid at the lowest point. Lift a ceiling tile up off the grid. Tilt it and lower it down through the grid opening. Repeat this step with the remaining tiles in the immediate area.
Measure the distance from the ceiling grid to a ceiling joist in a location that is not sagging. Add six inches to this measurement. Cut two to three 16-gauge hanger wires to this length, using tin snips.
Screw a small eye bolt into the ceiling joist above the sagging area. Insert one of the 16-gauge wire's ends approximately three inches through the eye bolt. Bend the wire down and wrap it around itself three to four times.
Push the ceiling grid up gently until it is lined up with the laser level. Notice where the ceiling grid intersects the hanger wire you just hung. Gently lower the ceiling grid and bend the wire at a 90-degree angle where the grid has intersected it.
Push the grid back up and insert the wire through the nearest runner hole. Twist the wire around itself three to four times.
Use the laser level to determine whether the wire fixed the sagging or if the ceiling grid needs more support. If you need to add more wires, repeat Steps 3 through 5. Tilt the ceiling tiles to fit them back above the ceiling grid and lower them in place.