Britt Bravo has a great blogpost with 5 tips to start a nonprofit blog. Shortened from Britt's blogpost, so click on the link if you want to read the full explanation of the 5 tips:
1. Read blogs.
Before your organization starts to blog, set up a newsreader, whether it is Bloglines or Google Reader or something else, and see what is being written about your organization and the issues that it represents. Not only will this give you a feel for the different styles of blogs, but it will also provide content for some of your first blog posts.
2. The best person to write an organization's blog is the person who is the most excited to write it.
You need a staff person who is not only excited to write on a regular basis, but also wants to immerse themselves in the "blogosphere."
3. Post consistently.
There are all kinds of theories about how often to post on your blog. The most important thing is to be consistent. You don't have to write every day, but once a week is good. T
4. Have an RSS feed and comments.
In my opinion, a blog without an RSS feed or comments is not a blog, it is simply someone writing regularly on a web site. A blog allows interaction through comments, and an RSS feed allows readers to subscribe. For your less tech savvy readers, you should also allow your supporters to subscribe to your blog via email with a service like Feedblitz.
5. Just start.
All very good and recognisable tips!