I was able to reuse my original headliner after cleaning it with Simple Green and it turned out perfectly. Using the poster board and gluing the headliner to it makes this a 1 man job.
I used my headliner as a template to cut the poster board as the original indentations where the trim panels fit were clearly visible, which made things even easier.
Here is Pantera 4134's method:
"Like most of you, the need for repairing our fallen headliner is one of those projects that sooner or later we all tackle. I have read all of the posts and gotten some good advice. Various ideas from gluing to a plastic board or directly to the roof were considered. A local trim shop here in Atlanta gave me an idea that worked for me and was easy and inexpensive.
At an office supply store I purchased a white foam display board (40x60x3/16) it is two pieces of poster board with a foam inner piece. It is also light weight, flexible, and easy to cut. After removing what was left of the old foam from the headliner with Simple Green and scraping the roof with a plastic scraper (also a shop vac works well) I was ready to fit the foam board. I rough cut the board with a utility knife until it fit with in the cavity provided by the side supports and front braces. Next I used 3M 80/90 trim adhesive. (NAPA)This is an adhesive used for vinyl tops or heavy headliner installations. I glued the headliner to the board. After 24 hours to assure good strength I reinstalled the board. The board will stay up by the trim panels and could be easily removed if necessary. The cost of this was about ten bucks for the board and another ten for the adhesive. Time wise it took about three hours. I think the foam will be of some help as an insulator and sound deadener."