The Developer Tutorials blog has posted their "winner" of a list of things to do to keep you and your website secure:

When it comes to PHP security, you often think of input filtering, SQL injection prevention, XSS in user submitted content and so on. Well, forget about that. They're all pretty trivial compared to what I feel is by far the most important security issue of any web application. Read on to find out why.

Their number one thing to do to keep safe and secure? "Keep your systems updated." They point out that security a web site is one thing but securing the system that it runs on/has been developed on is another (and improperly patched, it could cause some huge problems).

Now, the best way to keep on top of potential vulnerabilities in your underlying systems is with updates. Sometimes, however, you need to go a little further. [...] Watching the web is also a good idea; major security vulnerabilities generally make it to the front page of Digg.