I started out exploring because, with this particular mask and headgear, where I needed to put the headgear to secure the mask properly, there was nothing to stick the velcro to, so I'd end up with the velcro grabbing things I didn't want it to grab and had this big jumbled mess of stuck-together straps to fight with in the dark.
So I thought of filling in with some fabric for the velcro to stick to and found that a nubby washcloth seemed to be the best for grabbing it. I just crudely sewed it to the headgear. (Washcloths don't play nice with sewing machines, so it doesn't look very pretty on the bottom side, but I didn't want to mess with stabilizers or things that might have looked pretty.)
So now I have a place to put my velcro in between the straps, which is cool. BUT...the biggest benefit was that, for the first gosh-darned time since I started all this, the mask stayed where it was supposed to for the entire night. Usually, around 2 to 3 o'clock, I can count on the seal breaking and having to start fighting to get it back in place. And always, when I roll on my back, the masks will start honking at me as my face relaxes at the chin. Last night, despite being oily, sweaty, drooly under that mask, it stayed quiet!! So unless it was one night's fluke, I think maybe the washcloth grabs your hair just enough to keep the mask from sliding around so much. It's not that there weren't slippery things on my face for a change -- if I'd grabbed it and slid it, it would have happily gone for a ride. But this mask usually rides up my nose at night. None of that. Always honks at me at some point in the night. None of that either.
So just in case anyone else wants to try it (and you wouldn't even need a sewing machine; you could hand-baste it, I think), here's a picture. Nothing fancy, but for me it made a huge difference.

Peggy