I don't have one. I usually look to the left, and behind, but can also look to the right and behind as needed.

There is a tendency for the bike to pull in the direction one is looking, which may take some practice to overcome.

I tried a bar mounted mirror for a few minutes, but I figured that was easier just to look behind myself than look down at the bars. I have considered a helmet mounted mirror, but haven't quite found the right one.