Starting postmaster in rc.local

On the following page

http://www.postgresql.org/idocs/index.php?installation.html

It says that I need to su to the user postgres to start postmaster.
In fact, when I try to start it as user root, it won't let me and that
is somewhat understandable. But ideally, I'd like it so that user
postgres does not have a shell (which is what I've done for the
mysql user I had to set up).
When I tried to set up the command to start up postgres in the
rc.local to run as user postgres, I'm having problems. I know there
has got to be a way for me to be able to do this but I'm still too
new to *nix to know how. I've looked throughout the PgSQL docs,
mimiced the line I have for mysql (but making the relevant changes),
did some searches on the web but have come up blank.
Could someone point me to where I need to look to discover how
to do what I want?

thnx,
Chris