initdb /usr/local/var/postgres -E utf8    # create a database cluster
postgres -D /usr/local/var/postgres       # serve that database
PGDATA=/usr/local/var/postgres postgres   # …alternatively

If builds of PostgreSQL 9 are failing and you have version 8.x installed,
you may need to remove the previous version first. See:
  https://github.com/mxcl/homebrew/issues/issue/2510

To migrate existing data from a previous major version (pre-9.3) of PostgreSQL, see:
  http://www.postgresql.org/docs/9.3/static/upgrading.html

When installing the postgres gem, including ARCHFLAGS is recommended:
  ARCHFLAGS="-arch x86_64" gem install pg

To install gems without sudo, see the Homebrew wiki.

To have launchd start postgresql at login:
    ln -sfv /usr/local/opt/postgresql/*.plist ~/Library/LaunchAgents
Then to load postgresql now:
    launchctl load ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist
Or, if you don't want/need launchctl, you can just run:
    pg_ctl -D /usr/local/var/postgres -l /usr/local/var/postgres/server.log start