Steps to install and run PostgreSQL using Homebrew (Mac OS X)

launchctl unload -w ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist

Rename the old version and update PostgreSQL

mv /usr/local/var/postgres /usr/local/var/postgres91
brew update
brew upgrade postgresql

Init PostgreSQL

initdb /usr/local/var/postgres -E utf8

Update old DBs

pg_upgrade -b /usr/local/Cellar/postgresql/<old.version>/bin -B /usr/local/Cellar/postgresql/<new.version>/bin -d /usr/local/var/postgres91 -D /usr/local/var/postgres

Copy Homebrew agent and start server

cp /usr/local/Cellar/postgresql/<version>/homebrew.mxcl.postgresql.plist ~/Library/LaunchAgents/
pg_ctl -D /usr/local/var/postgres -l /usr/local/var/postgres/server.log start

Stop it manually

pg_ctl -D /usr/local/var/postgres stop -s -m fast

Start/Stop via launchd configuration

launchctl unload ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist
launchctl load ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist

Create new DB

createdb mydb

More Info

How to start PostgreSQL server on Mac OS X?