I used some "headliner adhesive glue" from Classic industries (I think). maybe it was in the Paddock catalog. Anyway, I was a bit drunk when I slathered that stuff on the underside of my new headliner. That glue will get you HIGH

It holds awesome... but beware: it will eat paint! I had the headliner on the hood and some glue got on the fender...
EDIT: Once you put the headliner up... Put constant, even pressure on the headliner. use 3-4 2x4's on the seat to the top to hold the headliner overnight while the glue sets...