A new book by Tanni Haas interviews 20 political bloggers and gets their thoughts. Here is one from Tyler Cowen on how to have a successful blog:
It needs to be updated regularly and to some extent be self-critical or self-reflective. And the person should read widely on the Web. That's a requirement. There are some very smart bloggers who shall remain nameless. They don't read other blogs or respond to them, and I find what they produce to be a little bit backwards. So it's important to be on the frontier of the debate in the political blogosphere.
And Matt Yglesias:
It's especially important, in terms of quality, to ask why people would be reading it. What do you know that other bloggers don't know? What value is being provided? This is especially important if you're new and don't have an institution backing you. People are instinctively going to be a little bit wary. So you really have to find subjects you're knowledgeable about, and deliver information that, if other people link to you, they don't wind up looking like idiots.
The book is here. See also Cowen’s thoughts here.