Design a user friendly website with clear navigation and clean code. The clean code can be achived by using div based (tableless) html and css. Validate the xhtml and css with w3c standards.
Regularly update content on the site to keep the site fresh. For corporate sites, use news and blogs to update new and relevant content on the site.
Use keywords judiciously with relevance. Use unique set of keywords on each page based on the content of the page. Use the same keywords and phrases in page title, meta content, link text, image ALT attributes and page URL.
Create a blog on the site and regularly blog about new products, services, news and experiences. The blog content provides insight to the prospective customers about your website as well as provides fresh content to search engines.