git clone git@github.com:YOUR-USERNAME/YOUR-FORKED-REPO.git
cd into/cloned/fork-repo
git remote add upstream git://github.com/ORIGINAL-DEV-USERNAME/REPO-YOU-FORKED-FROM.git
git fetch upstream
git checkout master
git pull upstream master
So that any commits of yours that aren't already in upstream/master are replayed on top of that other branch:
git rebase upstream/master
If you've rebased your branch onto upstream/master you may need to on GitHub. You'd do that with:
git push -f origin master