Left arm above the wrist joint with the watch face inward. Meaning the face is towards the body. It protects the face from hitting things and getting scratched but also since my watch is "very" luminous - don't want the enemy to see it at night. Or at least that was why some years ago - now its just habit. Plus when you are sitting in a long boring meeting its easier to read the watch without conspicuously turning your wrist over.
I back a few of my watches with clear resin half-domes (self-adhesive) available at craft stores. They're actually the "glass" for miniature clocks. They make the flat back on most watches more comfortable and also snug up the fit just a bit. Cheap, too, they come in a six pack.
That's the way my dad wore his watch, as did I when I still wore one. His reasoning for the "reverse" orientation was, as a truck driver, he could look at it without having to take his hand off the steering wheel. My reason was simply because that was how dad did it.
