What are ways for graduate students to make additional money?
Finish the PhD quickly.  People with PhDs are paid more than people without PhDs.  Becoming a PhD faster will get you to higher pay faster.  You will also keep that pay for longer, because you will work more years of your life if you finish your education sooner.  (note:  correlation between education and pay does not always imply causation; also in certain disciplines the pay is terrible both with and without a PhD)
Quitting the PhD to go into industry is a good choice if making money in the short term is the goal.
Switch to a university which provides decent funding.  This works best if you are early in your PhD.
Apply for outside fellowships.  However, keep in mind none of these pay that great.
Working a job unrelated to the PhD while enrolled will make the PhD take longer, delaying the time when you are paid more.  Do not do this if you can avoid it.