Keep your Forks in sync (Source: https://github.com/rilwis/meta-box/wiki/X.01.-Keep-your-Forks-in-sync)

Keep your Forks in sync

# In your local clone of your forked repository, you can add the original GitHub repository as a "remote". 
# ("Remotes" are like nicknames for the URLs of repositories - origin is one, for example.) 
# Then you can fetch all the branches from that upstream repository, and rebase your work to continue 
# working on the upstream version. In terms of commands that might look like:

# Add the remote, call it "upstream":

git remote add upstream git://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.

# 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

# You only need to use the -f the first time after you've rebased.