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What do you know?

If you want to write about what you know, do it. Even if someone else has written about it, you may write about it differently. Maybe your "voice" will reach people that didn't like another blog post about the same subject.

If you decide, as saluki suggested to write about something you don't know; do as he suggest and provide details and references. This could be more interactive, if people are trying to learn along with you.

The key is to write -- both on schedule and as well as you can. Adhering to a schedule means people can easily follow along and know when you are posting. The more you write, the better you get.