My clients will tell you that I am always preaching the importance of site architecture (or information architecture). Your site’s architecture, links, and navigation communicate how information is related within your site. Each page in your site passes rank to the pages beneath it. If your site lacks organization, Google must rely on other factors to determine the importance and relationship of your pages.
WordPress and SEO guru Joost de Valk has a great write-up on this topic. http://yoast.com/site-structure-seo
Google’s Matt Cutts discusses site architecture here: http://www.stonetemple.com/articles/interview-matt-cutts-012510.shtml
I encourage new clients or anyone wanting to improve their rank to start with a Website Audit. The audit examines the way people you use your site, the keywords and topic of the highest importance and makes recommendations for what an optimized site structure would be.