I have a small mirror that mounts onto the side temple of my sunglasses. I can't recall the brand - but it was some sort of small company, perhaps a one-person operation. It has a mirror that is roughly 1" by 1.5", and the mirror angle can be adjusted in both the x and y directions. But once you get it adjusted, it stays in place.

I would say it takes one or two rides to get used to moving your head slightly to "aim" the mirror. But once you get used to doing this, it works far better than something that is fixed to your bike.

My other mirror "rule" is to never totally rely on it. If I see a car in the mirror then I know I can't turn left, or otherwise move out into the roadway. But if it looks clear in the mirror, I still "double check" with a look over my shoulder.

I also find it helpful in knowing if a faster rider is coming up behind me, so that I give a bit more room.

Later add: It turns out my mirror is a "Take A Look" mirror.