Say, you have an existing octopress blog published on github pages, here is how to configure it:
git clone https://github.com/username/username.github.io.git
cd username.github.io.git
git checkout source
mkdir _deploy
cd _deploy
git init
git remote add -t master -f origin https://github.com/username/username.github.io.git
rake gen_deploy