It is certainly tempting to try to box up SEO as another project with a beginning, middle, and an end. However, the important stuff never ends. As you can see in this report by SEOmoz on search engine ranking factors, external links (links to your site from other places) remain the gold standard for improving organic search engine rankings on your site.
The first three factors all relate to backlinks:
- Keyword Focused Anchor Text from External Links
- External Link Popularity (quantity/quality of external links
- Diversity of Link Sources (links from many unique root domains)
What this means is that any successfully "SEO'd" site not only must be optimized for SEO and your target keyphrases, but must also (a) conduct an ongoing campaign to build inbound links from other sources; and (b) be consistently creating new content that third parties would want to proactively link TO.
Lawrence W. Gallick