Bartending, waitressing, and or working as a cocktail waitress can earn you a lot of tips, especially in high trafficked or high end restaurants or bars (or in popular vacation cities). Nannying can get you anywhere from $10-$20/hour for more affluent families, and some families even offer benefits and paid weeks off for good nannies.
Getting a research grant or some kind of educational stipend can be helpful too because you usually get a lump sump or the ability to earn a significant amount of money, and can spread the work load in between classes, the weekends, etc.
Lastly, I'd recommend doing brand ambassador work as a promotional person. you usually just hand out postcards or product at events or on campus and you can make really good money (like $15-$20/hour) doing it.