Google has released a Search Engine Optimization Starter Guide (pdf) that covers around a dozen common areas that webmasters might consider optimizing. We felt that these areas (like improving title and description meta tags, URL structure, site navigation, content creation, anchor text, and more) would apply to webmasters of all experience levels and sites of all sizes and types. Throughout the guide, we also worked in many illustrations, pitfalls to avoid, and links to other resources that help expand our explanation of the topics. We plan on updating the guide at regular intervals with new optimization suggestions and to keep the technical advice current.
Also, check Google’s list of best practices.