Be bold. Use the <b> </b> tags around some of your keywords on each page. Do NOT use them everywhere the keyword appears. Once or twice is plenty.
Deep linking. Make sure you have links coming in to as many pages as possible.
Social bookmarking. Make it easy for your visitors to social bookmark your website, creating important links that the search engines value.
First come, first served. If you must have image links in your navigation bar, include also text links.
Multiple domains. If you have several topics that could each support their own website, it might be worth having multiple domains.
Article exchanges
Site map. A big site needs a site map, which should be linked to from every page on the site