I tried a visor clip, but the sunglasses hung down too low. Finally, I came up with this:
A couple of rubber self-adhesive pads from the hardware store on the top of the visor.
The glasses fold over the visor where you may normally try and store them, but the pads against the arms keep them from sliding off.
The visor is in normal position against the headliner and the glasses aren't going anywhere until I pull the visor down.