What is the most important SEO activity for a new blog?

[Brian]

Assuming that you've covered the disaster basics (you're site isn't noindexed, and you aren't copying content from somewhere else) the number one factor in SEO is links from other sites pointing to your site. The internet is filled with linkbuilding advice and places that will charge you for it. Unless you really, really know what you're doing, do not pay for links.

The easiest way to start spreading word about your blog is to participate in community forums and blogs closely related to your subject. Don't push your blog, but instead participate in meaningful conversation and leave you blog address as your signature. You want to get people talking about your site and recommending it themselves.

You can also benefit from social media: become active in Twitter and Google+ -- don't just set it up and leave, but be active. Follow others, participate in conversations. If you can make some simple videos, that's another fantastic way of promoting your site (YouTube is the second largest search engine).

You'll want to cover the on-site SEO basics -- titles, keywords, images, alt tags -- but the majority of SEO benefit comes from links. You can learn a lot about SEO at http://www.seomoz.org