Here's how I make a backup of a single Postgresql database using the pg_dump command:

pg_dump -h db_server -U db_user -W db_name > db.20070725.dump
Password:

With this command I'm doing the following:

• Connecting to a server named "db_server".
• Connecting as the user "db_user".
• Forcing a password prompt with "-W".
• Dumping the database named "db_name".
• Sending the output to a file named "db.20070725.dump".

As a final note, here is a link to some "backup and restore" documentation at the postgresql.org web site.