I know at a previous job I was one of the people being reticent about posting blog entries, largely because it's kind of big and intimidating to think posting your own thoughts for public, uncontrolled consumption if you're not used to it—and sometimes even if you are. That, and I know a lot of people who don't think they have anything to talk about that hasn't been covered before.

I think it helps to approach people with the seed of something, like, "a walkthrough of that big feature you implemented to explain why it's a big deal to laypeople" or "write-up of how we made the decision to pick this technology over that technology." The way I was initially sold on doing conference talks might also help, here, too, for people who've already written stuff but can't be persuaded to post it—just because you're not the only one who can talk about a subject doesn't mean that you can't talk about it in a uniquely useful or engaging way.