Ninety-nine percent of my sailing has been single-handed...makes it easy to find the guy at fault but hard to win arguments... Common sense is the main rule, but there's a couple items I stand by hard and fast when out alone.
When conditions require (sea state, darkness, etc.) I snap on to the jackline on the way out the companionway.
Seacocks, fuel valves, 'lectric breakers, etc. are ALL off or closed when not in active use...no exceptions.
I have about a 200' length of 3/16" line with a little line float on the end hooked to the trip on the steering vane...it is ALWAYS deployed while offshore. As a single-hander that line is a very important safety feature...if I do inadvertently go in the wet, figure I can make it to that line and hit the trip before she heads off over the horizon like the "Mary Celeste."