I’m sure there are better ways, but I’ve had great success with a wire wheel on a variable speed drill.

Thanks Eddie, sometimes I've used this same process, but sometimes it works better, and others doesn't work so good.

We use a dual action orbital sander with 120 grit sandpaper. Holding it against the board at a slight angle, we pull the sander towards us cutting under the old foam. This works very well to pull large chunks off of smooths surfaces. Then we use the wire wheel to finish in the tight spaces.

If it’s a sticky mess I’ll use baking soda and a tire brush. I’ll normally use a sander with 36 grit. In the tight creases and curves I’ll use a “stripping cup”

Thanks for all your answer!
Jesse, the baking soda that you use in case of sticky mess, i imagine it's in dust format... Doesn't it?

No I use the baking soda when it’s stickers balls and smears. It’s usually the euro cars that are sticky, but I had a crossfire that was a mess.

I have had good luck with just air nozzle, air just gets under the foam and peels it off, go over with a scotch pad, blow it off again and done.

I use the orbital sander method on foam that is not quite rotted out, wire brush on the harder headliner boards. My magic way to get those Import headliners super clean in denatured alcohol, put it in a spray bottle , after scraping the best you can with a hand wire brush saturate a couple square feet at a time and wipe clean with an old towel cut up into rags. Also use it to clean up the sunroof tray area. By spraying in and use an old scrap of foam headliner rolled up and taped to metal yard stick.
Be sure to cover all the seats in case of a drip. I use it to get any foam residue that had turned to a greasy type mess. Good luck!