I take it slow, and use a higher gear to avoid spinning. I also avoid overtaking since accelerating spins the back wheel out of control.
When there are thick, unavoidable, patches of ice I test the back brake, very softly, to see how much traction I have. If the back wheel glides with the slightest push I let the bike slow down enough to continue. Putting the feet down in such occasions help.
But generally when my brakes get ineffective from the ice I stop riding, and get the car that has 4 wheels and tyres with studs.