Remove old Drush

If you already have an installation of Drush on your machine, go ahead and get rid of it. If you installed it with homebrew, simply run:

brew uninstall drush

Install Composer

You most likely have composer on your system already, but just in case you don't, just run this:

curl -sS https://getcomposer.org/installer | php && mv composer.phar /usr/local/bin/composer

Install Drush

This is a two step process. First you install drush with brew, and ensure that it is at the trunk version of the drush repo. Then, you'll need to let let composer install in drush's root.

1. Get drush from homebrew

brew install --HEAD drush

After that command has run, check the drush version:

brew info drush

If it's still on 6.x (which it shouldn't be), check to the HEAD version by running:

brew switch drush HEAD

2. Composer Install

cd /usr/local/Cellar/drush/HEAD/libexec && composer install

Check your work!

Restart terminal, and then run

drush --version

. The output should be:

Drush Version   :  7.0-dev

Good luck!