I've heard of people giving it a tiny little clockwise turn, just to break it loose, then turning counter clockwise to undo them. The only thing that concerns me is that they are plastic or bakelite, so they may not take to that very well.
The other thing that comes to mind is pushing down a little while you turn them CCW.
You do have a good screwdriver that fits really well, right? A good screwdriver makes all the difference.