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.
Use search engine friendly URLs and use hierarchical strcuture. For example, all the pages under Services section can have url: sitename.com/services/servicename.
Use qualitative link building for the promoting the site. The link building should be with relevant quality websites.
Use Social Media marketing efficiently. Use social bookmarking to popularize your content and services on various social media engines like Twitter, Facebook, Stumble, Delicious etc.
Create a sitemap on the site for the visitors as well as create an XML sitemap for the search engine crawlers.
Create a proper 404 error page which can be linked to the sitemap so that visitors dont leave the site.
Use RSS feeds and newsletters for building long terms loyalty with your customers.