I'm bookmarking this as an essential. I'm surprised I haven't read this post before by Merlin Mann, but then it was a long time ago.

Essential advice for bloggers.

Good blogs reflect focused obsessions. People start real blogs because they think about something a lot. Maybe even five things. But, their brain so overflows with curiosity about a family of topics that they can't stop reading and writing about it. They make and consume smart forebrain porn. So: where do this person's obsessions take them?

— What Makes for a Good Blog by Merlin Mann

Hat tip to Patrick Rhone for this.