Symlink error when installing MySQL via Homebrew

Trying to install MySQL via Homebrew. The install seems to work fine but i get an error:

"Error: The linking step did not complete successfully
The formula built, but is not symlinked into /usr/local
You can try again using `brew link mysql'"

Naturally, after this I ran:

brew link mysql

Which spat out:

Error: Could not symlink file: /usr/local/Cellar/mysql/5.5.20/include/typelib.h
/usr/local/include is not writable. You should change its permissions.

So I ran it with sudo and got a "cowardly refusing to brew link mysql".

Obviously that folder doesn't belong to you. Just try with:

sudo chown -R $(whoami) /usr/local/include

You never have to use brew with sudo. If you do, something is wrong with your installation.

Then, try the link command again.

slhck's answer is great, but I think it is a little cleaner if you are a member of the admin group (probably are if you are using Homebrew) to make all locations in /usr/local writable to group.

sudo chmod -R g+w /usr/local