I came up with a couple of ideas for how to tighten up the headliner on my '89 740 GLE, until I can re-do it properly. For the edges, I used 1/8" wooden dowels, about 1' long, pressed into the space between the headliner and the trim. For the sag in the back, I used 8 white plastic panel fasteners pressed into the sides of the recess over the rear seat. The 1/8" dowel was $.63 for 48". That was enough to do over both front doors, and the windshield. The panel fasteners were $.55 each. Total cost was about $5, including tax. Time to install was less than half an hour! Cheap and easy!
http://s177.photobucket.com/user/Ronzol61/story/70613

 

Better yet: go to a junkyard, pull a late 740,940 or 960 headliner and install it! PickNPull charges $15 for the piece and the job looks the way it should. Yes, it takes a little more work!

 

Lev, that would be a fine idea, if we weren't in the deep freeze here in New England! I'll look into that when the weather warms up a bit!