Top 20 SEO Factors from the Search Engine Ranking Factors Guide by SEOmoz.
Each principle has commentary by SEO experts. You can learn a lot by reading the comments. Some of these factors work better in some search engines than others (like misspellings in meta keywords work in Yahoo but not many other search engines – though perfect to use in your PPC ads). Some of the factors I've completely ignored or don't quite know what they are speaking of.
My biggest SEO lesson. Your title tag, and H1 header tags should match exactly and have the main keyword for the page. I changed my title tags on a site, it lost page rank and it has never returned (big regret that I can't change it back!).
The top 10 positive factors – what you should be doing (in order of importance)
1. Use keyword in title tag (can you believe this simple factor can have the biggest benefit?)
2. Global link popularity of site (how well linked is your site?)
3. Anchor text of links going to your site (use your keyword as the text that is linked, rather than making all links say “click here”)
4. Link popularity within the site's internal link structure (not sure what this means)
5. Age of site (unlike people, search engines favor the old. It shows a sense of permanency and therefore greater trust & higher ranking)
6. Topical Relevance of Inbound Links to Site (if your site is about internet marketing are your links also about internet marketing and related terms?)
7. Link popularity of site in topical community (so if your site is about affiliate marketing, get more links from other affiliate marketing sites)
8. Keyword use in body text (repeat your keyword a few times in the body of your web site, especially as links, bold, headers, or italics).
9. Global link popularity of linking site (get links to your site from authority sites, like .edu sites, Wikipedia, high page rank, etc)
10. Topical relationship of linking page (just meaning does that particular page that links to your site relevant?)
5 negative SEO factors to avoid:
1. Server is often inaccessible to bots
2. Content very similar or duplicate of existing content in the search engine
3. Your site links to low quality or spam sites
4. You participate in link schemes or you actively sell links
5. Duplicate title/meta tags on many pages (mix them up, a new title and meta tag for each page on your site)