# Add the remote, call it "upstream":

git remote add upstream https://github.com/whoever/whatever.git

# Fetch all the branches of that remote into remote-tracking branches,
# such as upstream/master:

git fetch upstream

# Make sure that you're on your master branch:

git checkout master

# Rewrite your master branch 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 don't want to rewrite the history of your master branch, (for example because other people may have cloned it) then you should #replace the last command with git merge upstream/master. However, for making further pull requests that are as clean as possible, it's #probably better to rebase.

git merge upstream/master

#Update: If you've rebased your branch onto upstream/master you may need to force the push in order to push it to your own forked #repository on GitHub. You'd do that with:

git push -f origin master