I use this:
Efficient Velo Tools Safe Zone Helmet Mirror (Black) - Performance Bicycle (performancebike.com)
It gives me a good view of what's coming up behind me, and allows me to just angle my head a bit to see regardless of my position on the bars.
Prior to this, I used one of the metal mirrors that clipped onto my glasses. I did have a crash, and the mirror broke off. The metal part that sticks out to hold the mirrror impaled itself in my helmet liner. Another inch and it would have impaled my temple or eye. The EVT mirror will snap off in the event of a crash and is easily snapped back on.
I use the mirror in conjunction with my Varia. When I get a Varia notification, I check the mirror to see if the driver is moving over to safely pass. In my experience, they will move over long before they get too close to me. If not, I'll look for a bailout spot. I've been hit once, don't want to go through that again.