One of the primary reasons that companies outsource to SEO services firms is the knowledge they bring to the SEO copywriting process. Sites that are poorly written from an SEO standpoint face an uphill battle in gaining position at the forefront of search engine results.
To help you understand better the complexities of SEO copywriting, I've put together a comprehensive article on SEO Copywriting Tips for Google, Yahoo and Your Prospects. If you're committed to doing SEO on your own, this will give you an idea of what areas to focus on. If you're looking to outsource SEO, then use the article to determine which SEO services vendors are best capable to provide what your site will need to improve its search engine ranking.