Use valid, semantic, lean, and accessible markup

Most web browsers have advanced functionality to decipher the tag soup mess that is used instead of HTML on most current sites. You can't rely on search engine robots to do that to the same extent. Validate your HTML and avoid presentational markup - use as lean and clean markup as possible. By increasing your content-to-markup ratio, you make your site faster and more attractive to search engines.

High quality markup will help boost your rankings.

Accessibility is also very important. Making your site more accessible to vision impaired humans will also help search engine robots find their way around it. Remember, Google is blind, so even if you don't care about blind people using your site (which you should), you'll still want it to be accessible. This means that you should use real headings, paragraphs, and lists, and avoid using anything that may interfere with search engine spiders.

Remember that graphics, Flash, Shockwave, and JavaScript, although they can't be seen by Search Engines, are fine as long as they aren't required to navigate your site and to access vital information. Don't hide your content inside Flash files or behind funky JavaScript navigation. Browse your site in Lynx, and with graphics, CSS, JavaScript, and Flash off. If that gives you problems, it is likely to cause problems for search engine spiders.