How do I add the "proposed" repository?

I'm trying to either test a fix that's been proposed or I want to volunteer to test stable release updates before they are pushed out to general users, how can I enable the "proposed" repository?

From the wiki page on the -proposed repository:

To enable the proposed archive for Ubuntu 11.04 go to Applications→Ubuntu Software Center→Edit→Software Sources→Updates and ensure that natty-proposed is ticked.

Those directions map well to recent Ubuntu releases such as 10.10, 10.04, 9.10, etc.

Here are recent pictures from 12.04.

Using the Command Line

Manual addition

With root permissions edit your /etc/apt/sources.list:

sudo vim /etc/apt/sources.list

Then look for the following line or add it yourself:

deb http://archive.ubuntu.com/ubuntu/ precise-proposed restricted main multiverse universe

(If you are not using precise, you can replace it with your current version)

Save your file. Then run sudo apt-get update and install the package you want to use sudo apt-get install package/precise-proposed or sudo apt-get -t precise-proposed install package.

One liners

sudo sh -c "echo 'deb http://archive.ubuntu.com/ubuntu/ $(lsb_release -cs)-proposed restricted main multiverse universe' >> /etc/apt/sources.list.d/proposed-repositories.list"
sudo apt-get update
sudo apt-get -t precise-proposed install some-package

Done.