I know a fair amount about evangelism and a little bit about blogging, so I've combined the two to provide some insights into the evangelism of a blog.
1. Think "book," not "diary." A diary contains your spontaneous thoughts and feelings; you have no plans for others to read it. By contrast, if you write a book, from Day One you should be thinking about spreading the word about it. If you want to evangelize your blog, think of it as a product you're trying to sell and market the heck out of it.
2. Answer the little man. Imagine there's a little man sitting on your shoulder reading what you're writing. Every time you write an entry, he says, "So what? Who gives a shii-take?" If you can't answer the little man, then you don't have a good blog/product. It's tough to market crap, so make sure you have something worth saying.
3. Promote via e-mail. When I first started my blog, I sent out 10,000 e-mail announcements to all the people who had contacted my company in the previous nine years. Don't buy address lists or send spam to people, because for e-mail promotion to be effective, you must know the recipient, or the recipient must know you.
More e-mail tips: Whenever you answer an e-mail, mention your blog, and make sure your e-mail signature contains your blog's address.
4. Link to others. If I had to do it over again, I would have looked for interesting blogs that cover topics similar to mine. Then, I would have blog-rolled them all to ensure that Technorati pinged my blog so bloggers could find me. I use Blogrolling.com to create my current blog roll. I use NetNewsWire and Endo to look for new links to my blog, and I find sites that I would never have seen were it not for their links to my site.
5. Supplement other bloggers. Read the blogs of the top 50 bloggers (use Technorati's ranking) and see if you have in-depth knowledge about their topics. Then craft a real essay that complements the blogger's entry. One of the biggest challenges bloggers face is feeding the content beast. If you can help me feed it, I'll gladly link to you.
6. Acknowledge and respond to commenters. Only good things can happen when you read all the comments on your blog and respond to them. It makes commenters feel like they are part of your community and encourage others to read your blog.
7. Be bold. I'm not saying to intentionally piss off other bloggers, but if you can't speak your mind on your own blog, you might as well give up and stay on the porch. This is the fascinating thing about blogging: Even when people torch you, they link to your site.
8. Make it easy to join. I had no idea what Feedburner and Feed-blitz did until a blogger named Steve Nipper told me about them. Enable your readers to get to your blog in multiple ways. It's no different than distributing your physical products through multiple channels.