I work aboard ship and find it much more common than is generally thought.
A few tips
Atmosphere needs to be clear and portion of horizon where sun sets needs to be cloud-free.
Flash appears as a "bubble" on top of sun's upper limb (top edge) just as it's tangent to the horizon. Don't stare at the setting sun, I believe the brightness will interfere w/color perception. I do quick glances at the sun as it goes down, just as the sun's upper limb approaches tangency to the horizon I'll then watch. Sometimes I use binnoculars. Trick is not to start watching too soon.
Gatofeo's sighting makes sense, he wasn't staring at the sun prior to it happening.
Using celestial nav tables I've precomputed time and azimuth of sunrise then waited. A couple of times I've caught it then.
A couple of times I've seen it w/moonset (much more subtle) and once when Venus set.
Happy Watching, Kevin
Capt Jack, my nav-astronomy professor, would be amazed I've not only retained this stuff but still find it interesting.
Happy Watching, Kevin