Here's how I do it (assuming you're riding to the right of the lane - no crazy "ride/drive on the wrong side of the road, aka Britain/England):
-position right hand hold on an inboard position of the handlebar. Road bike: on top between stem and hoods, mtb/flat bars: covering brake mounts. This ensures minimal steering inputs keeping you tracking on your intended route.
-using left arm as a sight line, point towards the rear where you want to look
This works great for me when I want to be absolutely sure I know what's going on behind me at speed. The quick turn your head method is ok for slower speeds.
IMHO, mirrors whether bike or helmet mounted would just get broken by me, I'm pretty rough on stuff. That and I associate helmet mirrors with "Freds". lol!