Usually the only cleaning a conch shell needs is removal of the
periostracum (the brown fibrous coating on the outside).  They don't
require any polishing.  They have a natural sheen, unless they are old
beach-worn specimens, in which case no amount of polishing will make
them look good.  Liquid chlorine bleach is the usual means of removing
the periostracum, as well as any organic matter that remains inside the
shell.  Just take a plastic (not metal!) pail, fill with a solution of
1/4 bleach to 3/4 water (half and half works faster and is just as safe
for the shell, but you use up more bleach).  Lower the shell into the
solution, with the spire (the pointed end) down, so the solution can
flow all the way inside the shell.  Soak it overnight, or 24 hours if
necessary.  This will not affect the shell colors.  Take it out, rinse
thoroughly with water, inside and out, and you should have a shell that
looks just like the ones in the stores.  If there is a bit of
encrustation on the spire, you may have to pick this off with a metal
tool, but it is usually much easier to pick off after soaking in bleach.

Paul M.