How do I set datestyle to postgres at initialization?

I've been searching through all the newsgroups, archives, etc. and can't
find how to set the datestyle to "postgres" at initialization (versus the
default of "ISO").  I'm running 7.0.3 on RedHat 7.1.  When I try and use the
"pgdatestyle" setting I get an "Unrecognized variable pgdatestyle" error,
and "datestyle" is session specific.

Thanks!
-Ben