How To Prevent Malware on Lion & Snow Leopard Macs
Keep your software updated
Make sure your system is checking for software updates daily, not weekly. Updating your system software every day will ensure that the latest security holes are patched and sometimes even clean malware automatically, like with Apple's automatic tool for Flashback. To change update frequency, go to the Software Update pane, check off the box Check for updates, and choose Daily. I recommend installing just about every update unless there's a compelling reason not to in the description.