It's interesting how common these fish are nowadays. When I moved here back in '99 I think I managed to find 1-3 a year until 2004 came along. At that point I started finding a few more, then more yet the next year, and it's continued to where I pretty much expect to find flame angels when diving certain types of terrain.
I'm assuming there just are more these days rather than my developing an eye for them, although knowing what to look for does help with spottings.
If you are diving Kona and there's a dropoff with finger coral on it, keep your eyes peeled 10-15 feet ahead of you when you are diving roughly 50-60 feet. You almost have to spot them early, as they will dive into the coral if you get close or over them. Once you find them, try to remember exactly where they were, they often keep defined territories, often no bigger than a hula hoop, where you can find them over and over.
The five I ran across yesterday were not ones I knew locations of, although it's quite possible I've seen at least a couple of them before as I know I've seen a few in that area over the years. The corals were pretty non-descript so finding them again would be more a matter of diving at the same depth and hoping to spot them rather than knowing exactly where they are.
I chalk the increase in spottings for me to the collection management measures that were put into place back in January of 2000. Looks like numbers are improving nicely.