If you are like me you find yourself cloning a repo, making some proposed changes and then deciding to later contributing back using the GitHub Flow convention. Below is a set of instructions I've developed for myself on how to deal with this scenario and an explanation of why it matters based on jagregory's gist.
To follow GitHub flow you should really have created a fork initially as a public representation of the forked repository and the clone that instead. My understanding is that the typical setup would have your local repository pointing to your fork as origin and the original forked repository as upstream so that you can use these keywords in other git commands.
Clone some repo (you've probably already done this step)
git clone git@github...some-repo.git
Fork their repo on Github
In your local, rename your origin remote to upstream
git remote rename origin upstream
Add a new origin
git remote add origin git@github...my-fork
Fetch from new origin
git fetch origin
Set origin master
git branch --set-upstream master origin/master
git branch --set-upstream-to origin/master master
Push to fork (origin should be able to omitted given step 4)
git push origin