For people hacking on stuff using github repos:
Whenever you fork someone else's repo, make some changes to it, and then want to update your repo with the changes made upstream by the author, it's easy enough as:

# Add the remote, call it "upstream":  
git remote add upstream

# 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