I use a helmet mounted mirror (Take-A-Look clipped to the left side of the helmet visor) and consider it an important part of my safety toolkit in traffic.
I also recently switched from my Timbukt2 messenger bag to an Ortlieb cycling backpack.
The messenger bag blocked my mirror view directly behind me, when I'm in a low position. The backpack blocks my mirror view directly behind, all the time.
The only solution I've found so far is to adjust the mirror so that it points to the left, then rotate my head about 35 degrees to the left when I want to check my rear. With my head straight forwards, the mirror shows what's on my left-rear. With my head turned, the mirror shows what is behind me, and the motion move the mirror leftward just enough that I can see around the backpack.
This is much less convenient than just flicking my eyes up. I used to be able to ride while monitoring the mirror all the time (almost unconsciously). Now I am blind to my six o'clock until I consciously check it. Not pleased.
Has anyone found a better solution?