The simple answer is to never let anything go out of date. This is hard. Someone needs to look over the whole site every so often to make sure nothing's gone wrong, or every piece needs to be in someone's head so they know what needs updating when. Who's going to be that person?

But do we really need someone to do this? The web is the first medium where publication dates are entirely optional. Books, magazines, journals, newspapers, television, radio: they all had dates, or were simply ephemeral. Now you can update anything at any time, and the next visitor will see the new version with no hint of the old. That same freedom, however, imposes a cost: it makes people think your words are current.

In cases where you can, I think you actually do better to give up that freedom: put dates on things. Write blog posts when suitable and include "last modified" notes at the bottom of other pages. Let yourself write a snapshot and let the reader know that's what they're getting. This doesn't work for everything, but see if it applies before you set yourself up for maintaining something.

(Blogging also solves the problem of letting people keep on top of your changes. With a standard site you make small changes everywhere as things need adjusting, but there's no way for someone to follow along. With a blog they can check back for recent posts or use RSS. [1])

[1] Today is the end of Google Reader. It is a sad day, but we must carry on. I'm now on the gReader android app, backed by Feedly Cloud.