Upgrading to yosemite? If you aren't a developer then it's a snap. If however you've installed brew this can turn into an arduous task leaving you in limbo for hours. The steps to save you some time are really simple.
Step 1: backup via time machine
I always recommend this prior to any OS upgrade, just for CYA purposes
Step 2: move /usr/local
The yosemite install will hang for hours, mine even stopped. So before you install, just move it like so:
sudo mv /usr/local ~/.local
Step 3: install yosemite
from the app store, duh.
Step 4: move /usr/local back
sudo mv ~/.local /usr/local