git branch: gh-pages

More recent versions of git have an alternative to the git symbolic-ref method that Chandru explained. This avoids having to use the lower level commands.

git checkout --orphan gh-pages
git rm -rf .

You might find this tutorial useful:

Setup GitHub Pages "gh-pages" branch and "master" branch as subfolders of a parent project folder ("grandmaster").

To me this approach seems simpler then doing a git checkout gh-pages each time you want to edit your gh-pages content. Let me know what you think ^_^

Edit: I updated the tutorial link - thanks @Cawas. The old tutorial (not recommended) was https://gist.github.com/825950