After installing the latest & greatest:
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