Well, there currently aren't any viruses for the mac (OSX), and the only malware exists in the for of Microsoft Word Macros or stuff from OS 9.
"Word Macro: 553, Classic Mac: 26, OS X: Zero"
So if you don't us Microsoft office, or any classic applications, then all you really need up is your firewall and you should be fine.

If you're feeling extra paranoid, you can make sure that you don't regularly use the admin account on your computer (ie make a new account, assign it admin powers, log in as that account, take admin powers away from yourself, log back in as you). This will force you to "authorize" as the admin whenever certain hazardous actions are taken (adding applications, changing the OS, etc).

And then there are certain hacks, like the Terminal hack that suggests you should delete or rename Terminal to keep people from exploiting it easily. I'm not sure there's a list of 'em anywhere, but that's the state of the Mac...