Here's a snippet from an excellent article highlighting some simple but effective things to do to keep your Mac as safe and secure as possible. The link to the full article is below the quote.

The phrase "security through obscurity" gets tossed around from time to time when discussing Mac OS X. The theory is that since Macs still represent a fraction of the available computers on the internet, there's less of an incentive for virus writers, malware authors, spambot harvesters, Comcast sales reps, and other purveyors of electronic evil to harass and attack the platform. Why target 5 percent of the population when you can get much better results by going after Windows?

But the truth is that security through obscurity is a flawed idea. Yes, there are fewer recorded attacks on the Mac platform, but by no means does that make it secure. So what's a Mac user to do? Like the book says, don't panic. In this article we'll take a look at some basic ways you can improve your Mac's security right out of the box.

From Ars Technica [arstechnica.com]