In addition to making sure the letters in the the suffix of your callsign at
least minimize the possibilities of being mis-copied, there is also the "rhythm
factor" that I believe helps copy calls better on CW. Some calls seem to just
naturally "swing" and stick in my head better than others.
When I finally considered getting a vanity call after 40+ years of being W3GOI
(not only a lot of CW dits and dahs, but a mouth full on fone), I chose ones
that seemed to flow best in CW as well as not be mis-copied in the heat of a
contest. Also, I got tired of being a "3" in 4-land...
73 - John, W4AU