  We study the mean-field theory of charge-density wave (CDW) order in a
layered system, including the effect of the long-range Coulomb interaction and
of screening by uncondensed electrons. We particularly focus on the conditions
necessary for an ``extraordinary'' transition, in which the surface orders at a
higher temperature, and is more likely to be commensurate, than the bulk. We
interpret recent experiments on NaCCOC as indicating the presence of
commensurate CDW at the surface that is not present in the bulk. More
generally, we show that poor screening of the Coulomb interaction tends to
stabilize incommensurate order, possibly explaining why the CDW order in LSCO
and NbSe2 remains incommensurate to T -> 0, despite the small magnitude of the
incommensurability.
