Give your visitors the information they're looking for and make sure that other sites link to yours using Natural links. Also make your site easily accessible, every page should be reachable from at least one static text link.
According to Google: "Don't feel obligated to purchase a search engine optimization service. Some companies claim to "guarantee" high ranking for your site in Google's search results. While legitimate consulting firms can improve your site's flow and content, others employ deceptive tactics in an attempt to fool search engines. Be careful; if your domain is affiliated with one of these deceptive services, it could be banned from our index."