1. Pick a niche area. Make sure you know it in some depth.
2. Figure out what people in the niche want to know. This means reading relevant forums/threads/websites. Leave comments that count and link back to your site.
3. Become an expert. Don't just rehash what the other guys write about - that just makes you just one of 'em. Be an authority.
4. You'll also need the right tools. WordPress wins hands down as the best blogging software for SEO at the moment.