I use the CycleAware helmet-mount mirror. After decades of riding without a mirror, I wouldn't want to go without now as I feel it makes me safer on the road to confirm that the car back is moving over to pass (we don't have shoulders here). The mirror has an adjustable ball/socket base so it can be quickly turned down for photo-ops or easily removed for races.
It is designed to be mounted on the traffic side, slightly up and to the left of the straight-ahead field of view. This works well for me, as I'm right-eye dominant and don't really notice the mirror in the field of vision. But believe for a left-eyed rider it might be hard to get used to.