1.) Don't write about yourself. No one wants to hear about what you had for breakfast, or your cat's vet appointment.If you take this advice seriously, the first thing you'll notice is that most other writers don't.
2.) Don't write about writing. That appeals to other writers--not to your readers.
3.) Don't write about politics or other incendiary topics. That will alienate half or more of your audience.
There are highly successful writers (who are also highly successful bloggers), like David Brin and John Scalzi, who blog about whatever stuff they want, including politics. There are gobs of other great bloggers, like Michelle Davidson Argyle, Lindsey Buroker and Joe Konrath, who write about writing. There are others who blog on specialty interests which have nothing to do with the books. Deb Harkness had a wine blog; Jodi Meadows blogs about knitting. Davin Malasaran blogs about the important question What's Davin Eating? (It often includes updates about what his dog Peanut is eating too--sometimes the answer is books. See? Related to books after all.)
So, as usual the advice is only half right. Don't be too quick to dismiss that half, however. I think a good rule of thumb for blogs is the same as for novels: begin as you mean to go on. If you feel like having a blog about your breakfast, just remember, you have to maintain that. If you're going to talk about politics, you better have something insightful, not just something inciteful, to say. And do it regularly.
What doesn't work well is to have a blog about cooking and then randomly thrown a post in about writing, and then another post about the presidential debate. Your regular readers will be wondering, What is this crap? Consistency of topic is more important than what the topic is.
There's only one thing more important than consistency, and that's passion. Duh, right? It's great to do your research into the perfect blog that will attract 20,000 readers per day, but if you have no actual passion for that topic, you'll end up burning out quickly. And guess what? It's better to have a blog on a specialty topic that you post to frequently than a blog on a super-popular topic that... has no posts.
Finally, for fiction writers, we need to remember that blogging is not our main form of writing. My novels come first. If I have to neglect my blog for a week, a month or a year, to finish a novel, I'm not going to apologize for it. (We have all been there, right? "Sorry I haven't blogged in a while...")
So here are my three "rules" for a fun blog:
1.) Blog about something you love.What actually happens for me is that when I'm going strong on my novel, I'm also usually more interested writing posts for my blog too. When I'm too depressed to write my novel, I'm usually too depressed to write anything. Or do anything. (It's not pretty.) When I'm excited about writing, I want to not only write scenes but write about writing scenes.
2.) Begin as you mean to go on. Pick a main topic, plus a few related things you're willing to branch out to sometimes, and stick with that.
3.) Don't let your blog writing overshadow or squeeze out the time you spend on your other writing.
And that's why I write about writing. I know a lot of my readers aren't interested in the scaffolding behind the scenes, and I have no problem with that. But I don't see my blog as just a big advertisement for my books. Sure, it's part of the whole "doing social media" blah blah blah that writers, and everyone these days, is "supposed" to do. But if that were it's only purpose, I couldn't keep it up. I'm not much good at doing what I'm supposed to do. (This is one thing Dindi and I have in common.) I write about issues that are actually interesting to me. I give "writing advice" not because I think I'm such an expert (oh, I should have warned you about that) but because I am learning about it myself. As I figure something out (or think I have) I like to write about my process of discovery. Then you'll see some post about first chapters or subtext in dialogue pop up on my blog. ;)
Also, readers of mine who do read this blog... be happy if you see a lot of posts. It means I'm going strong on the next book....