Also, it's worth mentioning that a Mac isn't actually immune to malware. Mac viruses are almost never seen in the wild, and the only ones which have been seen required an ignorant user to enter an admin password, but that luck may not hold forever.
I work in the field of Mac technical support, and the only virus I've seen has been a harmless self-replicator which exploited Microsoft Word, but the Mac platform is definately getting more hacker attention lately than it's had in the past ten years from both white hats and black hats.
The best practices for keeping your computer secure are the same regardless of platform. You should use a secure and up to date web browser, such as Firefox with the aforementioned plug-ins, use a firewall, and be careful not to open suspicious files or click suspicious links. If you're ever prompted for your password, you'd better be opening an installer from a trusted source, otherwise close the window and delete the file.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.