I had to have my headliner removed by the dent people after a hail storm. I took the headliner, used 409, dawn soap, lots of water, then meguiars odor remover. Beleive it or not, it did not discolor my grey headliner and it made it look brand new. The odor remover was used to remove any 409 smell or dawn smell (im sure there wasnt cause i hosed it down really really well).

But anyway, the best way to clean it is to remove it and hose it down. let dry then install back.

I wish i had some before and after pictures, before it was a dark color and kinda tan (from smoking, etc.) now its grey/whitish.