First, standard jokes aside, just what is an "expert"??  To be concise,
it is someone other people think of when they need reliable knowledge
and/or skills in a particular field- someone others turn to for help!
The key word here is of course "reliable", as in consistant- a true
expert can of course be wrong sometimes, but he or she makes fewer
mistakes than most.  For example, the average identifier of molluscs in
my corner of the world-ocean may get 65% of the specimens in a given
sample correct.  An "expert" in this task might average 90-95% (Turridae
excepted, of course!!!)
        So, how does one BECOME an expert?  Mr Rinsberg has half of the answer-
experience.  One cannot become proficient at anything without a good
deal of experience (in conchology, this means examining many, many
shells!!).  The other half is the root of all science- *accurate*
observation and *correct* analysis or deductions!!  One may have all the
experience in the world, but  nothing can be gained until the brain is
shifted out of "neutral".  In the old "shell world", the most common
form of expertise is identifcation- differentiating often frustratingly
similar species using conchological features only (with locality and
habitat as useful but not definitve helps).  To do this reliably, one
must figure out the most "critical" or important charactaristics of any
given species- the one(s) which differentiate it from everything else it
could be confused with.  Since there is often considerable overlap in
features between closely-related spp (as in the current Chlamys
debate!), this is not always easy, and requires an open and agile mind
which doesn't see just what it expects, and doesn't mind at all when its
beliefs are contradicted by the facts!   Sometimes, this means looking
extremely closely- there are species in the genus Astarte, for example,
which can only be reliable told apart via a microscopic examination of
the periostracum!!  At other times, good analytical skills are needed-
Mytilus trossuls Gould, for instance, overlaps so thouroughly with M.
edulis L., that only an arcane set of ratios can tell them apart almost
100% of the time (they are so-called "crypto-species").  One can look
at  many millions of specimens, but unless the brain is engaged,
expertise is impossible.
        A further factor is Memory.  This doesn't nessessarily mean remembering
hordes of Latin and Greek words- these can be looked up in books- in
conchology it means remembering just what to look for, when, and how.
The best of experts will be able to cut to the chase and zero straight
in on the essential features of the situation- and not be distracted by
irrelevant details.
        One final thing which cannot be ignored, is the difference between
"skill" and "talent".  Expertise can be described as Applied Skill, but
some people are just naturally better at certain things than others- i
could practice writing music for the rest of ten lifetimes, but could
never be as good at it as Mozart or Brahms!  Not meaning to discourage
anyone, but in conchology as well as any other field, some people are
just naturally better at diffiicult ID problems, and especially at
FINDING shells in their natural habitat (or, say, in a pile of debris on
a trawler, in a short time-frame!).  THis does *not* mean that those
with less natural aptitude and predisposing abilities CAN'T become
highly skilled ("expert") at any particular activity, it just means they
will have to try a lot harder, and probably longer, to get to the same
level someone else seems to achieve with an irritatingly small amount of
effort.