Check out your school's work study program. If you apply for financial aid, you may receive the opportunity to participate in the school's work study program. Most college's work study programs allow the students to work at a variety of places on campus, like the library, for a limited amount of hours a week.
Even if you don't qualify for work study, you can still find a job on campus. Check out the library to see if they offer any non-work study work. Apply to your school's alumni association; most of them hire undergraduates to work the phones and collect donations from past graduates. If you know how to operate computers, than see if your school's IT help desk is hiring.
If you can write, then a number of online sources pay for freelance work. Freelancewritinggigs.com posts new online and offline writing jobs every day. Check out the various opportunities to earn some extra money while you also build your writing portfolio.
Many schools will pay students who excel at math, English, science and other subjects to tutor struggling students--especially athletes. If your school doesn't have a tutoring program, than start your own tutoring business using Facebook and word of mouth.
If none of these ideas work out for you, then apply for a job outside of campus. If there is a mall nearby, then apply at a store in the mall. Or try to get a job working at the local college hang-out spot. Every college town features college-oriented businesses, who are likely to accommodate your busy study schedule and give you time off during school breaks.