To manually stop or start a PostgreSQL 8.2 server that was installed via macports, use the following commands.
# sudo launchctl stop org.macports.postgresql82-server
# sudo launchctl start org.macports.postgresql82-server
Obviously this will also work for PostgreSQL 8.3 or 8.4 if you change the commands slightly.
I prefer the approach Linux distros have generally taken for service management, where all services are listed under the /etc/init.d diretory. To me /etc/init.d/postgresql-8.2 is easier, and you can use command completion to type it.