I had the same issue when I first bought my 02GT exactly where you're having it. Assuming your fabric is black, the easiest fix I can suggest is buying 4 (four) black screws and washers. I could not find any black washers, so I bought some spray paint and put about 3 coats on each side. I ended up only using 2 screws, it seemed to do the trick, but I still have two others if the headliner begins to sag again, requiring reinforcement.
When you're sitting in the backseat, carefully pull down your headliner to gain visibility, and you should see a metal bar about 1-2 inches wide running from one side of your car to the other.. about where the plastic panels on the back window are located. There should be a few inches offset from the roof itself, allowing you to drill holes into the bar and your headliner, then using the screws and washers, re-attach the headliner. Just be very careful when you're drilling the holes, so you don't add a roof repair to your list.
Alternatively, you can pull your whole headliner out, strip and re-do the adhesive (and the headliner, depending if the sag is due to warping), but it's a huge PITA. My headliner has held for 7 months and counting.