Ask HN: How do you protect your Mac?

A recent HN submission about how Apple has has pulled its "claims of virus immunity" and the subsequent HN discussion here: http://news.ycombinator.com/item?id=4156438 made me think of how I can better protect my computer from security threats. In the discussion linked above, reidmain makes an interesting argument that it is less about "hiding behind POS antivirus software" and more about raising user awareness of potential methods of security breach. I've tried so hard to teach my mom how to properly recognize shady websites, popups, downloads, etc. But often to no avail. So in my absence, what software/steps can my mom download/take to help prevent security breaches?