Check with your professors. They may need an intern to work with them, or they may know of some jobs that are available with other teachers.
Talk to your adviser. Let him know that you're looking for work on the weekends, and he may be able to steer you in the right direction. Be open; to get a weekend job you may need to work nights. Depending on your major, you may be able to find specialized positions--for example, working at a nursing home if you're in nursing.
Look at the bulletin boards posted around campus. They often include job postings, and you might find your ideal weekend job posted there.
Ask at various places on campus that might need you to work on weekends. Libraries are great places to work on campus, and student dining areas always need helpers. You can get to these places easily since they are on campus.
Start a business of your own, such as Avon, that you can do on the weekends, or check jobs on the Internet. For example, if you're an English or journalism major, there are plenty of writing jobs that you can do on the weekend.
Decide to put your assets to work for you by tutoring other students. If you're great in a subject, share your knowledge and get paid for it at the same time.