1. Use Google Sitemaps to see how Google sees your site, when it was last updated, whether you have any problems on your site, etc. I've written an article on this topic: How to Improve Site's Ranking with Google Sitemaps. You also want to provide a sitemap for your visitors.
2. Get involved in communities relevant to the content of your site. Visit their forums and mailing lists, and help other people by answering their questions, posting links to your site if they contain information relevant to your replies. Usually you are allowed to have a signature, where you can link to your site. However be aware that more and more sites implement a new link attribute rel="nofollow", which tells Google (MSN, Yahoo and other sites) to not count those links to your site's ranking credit. This is to avoid comment spam. You can find the details here: Official Google Blog: Preventing comment spam.
3. The head section of the document should include meta entries for keywords and description. Though it's been said that the keywords meta entry has little or no weight with Google, but is still useful with other search engines. Also make sure that the title of the document includes the most important keywords and phrases, as Google gives a heavy weight to those. The keywords need to be included in the H1 and H2 header entries, and also once in bold, once in italic and if possible in the URL.
4. Spell check your content. Google doesn't like when misspelled words are used, as it tries to auto-correct search words. Some sites use misspelled words to get more traffic to their site. e.g., "hign paying keywords" instead of "high paying keywords"
5. It's been said that sites containing valid XHTML are favored by the search engines. But it should at least use valid HTML. One other thing to make sure is that your site is readable by non-graphical browsers, such as Links and Lynx. Blind users use those to browse the Internet and search engines favor sites that are useful to more people. In fact search engines see your site as text, so things like javascript, DHTML and Flash may make it hard for the search engine to crawl your site.
6. Publish articles on other sites relevant to your expertise. Make sure that those articles link back to your site. I'm somewhat weary about submitting my articles to other sites, since then I end up with a duplication problem and a chance that a search engine would penalise duplicated content sites. Hopefully it somehow knows where the content has appeared in first place. But I don't want to take chances. So may be submitting unique articles which don't appear on my site is a much safer strategy.