As often as I look for this, I need to write it down. For example for my fork of github:dropping-spray, which got inspired by this tweet and the following discussion.

> # 1 - add the (so called) upstream repo so you can refer to it locally
> git remote add upstream git@github.com:Narigo/dropping-spray.git
> #2 - just double check it's there
> git remote -v
origin    git@github.com:wolframkriesing/dropping-spray.git (fetch)
origin    git@github.com:wolframkriesing/dropping-spray.git (push)
upstream    git@github.com:Narigo/dropping-spray.git (fetch)
upstream    git@github.com:Narigo/dropping-spray.git (push)
> # 3 - get up to date, I think in the flow as listed here its not needed, we are quite up to date
> git fetch upstream
> # 4 - fianlly merge in the latest stuff from the original repo
> git merge upstream/gh-pages

As you can see above, I merged the gh-pages branch and not master!