How to keep your forked GitHub repository up to date

1. Clone your fork:

git clone [e-mail]:YOUR-USERNAME/YOUR-FORKED-REPO.git

2. Add remote from original repository to your forked repository:

cd into/cloned/fork-repo
git remote add upstream git://github.com/ORIGINAL-DEV-USERNAME/REPO-YOU-FORKED-FROM.git
git fetch upstream

3. Updating your fork from original repo to keep up with their changes:

git pull upstream master

Source: this gist