I asked a professor if she knew of anyone looking for research assistants. It just so happened she was looking, and her work was interesting to me. We had an interview two days later, and I got the job.
A lot of people have recommended email, but I suggest you approach professors in person. Know something about their research and what you want out of a research role and start a conversation. If they don't have a spot for you, ask if they know someone who might. Professors get an endless stream of emails asking for research positions. A face-to-face conversation that shows you've done your homework is more likely to land you a position than blanketing the department with emails, IMO.