After a brief conversation with a friend on the subject, I present:
Elbeno's Technical Guide to Blogging Anonymously
I call it a technical guide because its purpose is to screen your tracks from prying ISPs, companies and other people who would employ Internet know-how to track you down. Obviously it's not going to protect your identity if you give away too many real-life details.
1. Choose a blog name. For best results, choose something totally unrelated to yourself. Something arbitrary. Nothing you are interested in. Look out of the window and pick two items, stick them together, you've got a name. e.g. dryice_squirrel. That's one I just made up. Don't use that one, or they'll know you know me. And if you have to add a number, don't add your birth year.
2. Download and set up Tor and Privoxy. They work for multiple platforms so you have some flexibility. They hide your IP address as well as current technology allows (which turns out to be quite acceptably well actually).
3. Turn on tor-enabled browsing now, and whenever you do anything related to your new blog identity. To make life easier, if you're using Firefox, you can install an extension that lets you easily switch between tor-enabled anonymous browsing and normal browsing.
4. Start building your anonymous identity with a GMail account under your new name. No, don't get an invite from someone you know, or worse, your existing GMail account. Google for one of the many GMail account spoolers around. If you require an email address to send the invite to, use http://www.mailinator.com/ for a throwaway mailbox (they handily give you an autogenerated random mailbox on the front page). At time of writing, there's a GMail spooler at http://kingsasquatch.com/. Use a brand new strong password. If you use a password (as many people do) that is one of your normal set, then it could be correlated in password files with any other GMail accounts you have.
5. Next step: create your blog, using your new identity and GMail account. You could probably do this on any old blog site, e.g. LiveJournal or Blogger. Use a brand new strong password, different from your GMail password. Google owns both GMail and Blogger. I don't think I need point out that data search and correlation is their strong suit.
6. You are now as anonymous as the Internet allows. Blog away. Rant about your work, your social situation, your politics. And always make sure you use tor-enabled browsing in any dealings with your blog or GMail account.
7. Don't ever tell anyone (friends, family, etc: people who know you) about your anonymous blog. Or it will no longer be anonymous. Even if they keep their mouth shut, it might suddenly have a lot of access from their IP address…