1. Clone your fork:
> git clone git@github.com:YOUR-USERNAME/YOUR-FORKED-REPO.git

2. Add remote from original repository in 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

4. Push your changes to remote
> git push