I have seen long discussions on various blogs and forums about SEO and various factors affecting it, but I have not found much to the effect of site layout on SEO. I think that this is the major factor among onsite optimization factors. The site must be very easy to use and read by the spiders as well as by the visitors. Just by visiting the main page of your site, your user must be able to see what your site is offering. If your site is some pages long, like 10 -15 pages, then this is very easy. You can use the left navigation bar for all the pages' links.

The name of each page must be very user and bot-friendly, like "how-to-improve-pagerank.html". This name will give you a good page rank. And users will also easily know what is on this page. If your site is made up of hundreds of pages, then you must group them under the suitable categories. Show all the categories on your home and all pages in left or right navigation. You must give the links of your main categories in the text of your home page, too. This will make your site well inter-connected.

All of your pages of the site must be reachable in a maximum of 3 clicks. This is the rule of thumb for good onsite optimization. Your site must also have a sitemap.html file that should list all the pages with links under the suitable categories. This will help users and will also help the search engines to index all of your pages.

Please, no flashy and heavy banners. These are usually annoying. Simple and minimum use of images is the best solution for the visitors.