This week, someone asked on the NY Tech Meetup mailing list: "What (are) some of the basic SEO optimizations and techniques you guys have used?"
Craig Wood posted a wonderful, succinct response. I'm reposting it below for future reference.
The only thing I would add: keywords in URLs and hostnames matter. Google ranks keyword-in-URL matches (and title) higher than just keyword-in-document. The closer this occurs to the start of the URL, the better. Hostname match is best of all (e.g. "newyork.craigslist.org")
Or, this was true the last time I saw any serious SEO analysis research, which is ~2 years ago. Maybe things have changed since then. :)
1) Titles - The biggest Bang you will ever have -- Never Duplicate your <title> tag
2) Common sense linking -- make all your URLs descriptive
3) Alt tags on Images
You do these three things well and you are 95% done with what you can do to get improve your ranking with the HTML of your site. There are all kinds of things you can do to tweak your content to be more towards culture references or 'hot searches'. And of course have a robust (no spam) external linking campaign.
-Craig