Fred,
What model do you have? If the headliner is like the liner in the W124 (smoother and vinyl-like) then any variety of cleaners should work. (more on that later) If it is slightly more "carpeted" like a W210 I would be more careful in what you use.

For light duty cleaning and universal protection use Lexol's Vinylex. If whatever is on there does not respond to that you can try Windex, failing that, 409/Simple Green. I would try and avoid the harsh cleaners if possible. Another option is the automotive creme "Goop". Apply to a particlularly nasty stain and let sit a couple mins. Then wipe with a damn rag. If yours is the slightly carpeted kind, then treat it like you would the rug/mats: vacuum, carpet shampoo, blot with a damp rag, blot with dry rag.

Whatever substance you use, scrub and remove with a terrycloth. The grain/nap will work its way into the texture better than a smooth rag.

Hope this helps....Lee