What are the key best SEO practices for a web site?

[latters]

Mainly large corporate sites. Also, howe do H1/H2 tags really effect a search ranking?

[Tom E]

Search ranking is predominately determined by traffic to your site and the number of incoming links found on other sites. To boost your page ranking make sure you are linking to sites that are relevant to your own. Linking to sites with high page ranking will in turn boost your page rank. Here is a free directory of link exchange web sites:http://www.linkenstein.net/