While we have written a bunch about the standard on-site SEO tactics, we have not devoted a ton of time to on-site architecture issues. By this I mean the navigational structure that you build for your website. I've been doing a bunch of reading and there is a large group of people that believe your navigation structure is highly related to SEO. Chad is always talking to me about this when we get a new client. He always wants to re-design their entire site ;-).
For a solid read, see this piece on Search Engine Journal. In it they talk about the importance of a relatively flat, but well-organized website structure. They caution not to build your website too deep or with any twists and turns. A good test is to think about the number of hops from the top of your website to the bottom. The fewer the better for SEO. I sat back and realized that our hop-count at Semify.com is fairly low. There really are not that many levels to our website. But there is a large amount of content. Perhaps that's why we are ranking well? They also mention that you should monitor your link count, stating that lower is generally better.
As I mentioned in my last on-site SEO tip blog post, there is always something new to learn in SEO. Such a great field for those who like technology, exploring and learning.